diff --git a/examples/circular.py b/examples/circular.py
new file mode 100644
index 00000000..21168368
--- /dev/null
+++ b/examples/circular.py
@@ -0,0 +1,18 @@
+import scadnano as sc
+import modifications as mod
+import dataclasses
+
+def create_design():
+    helices = [sc.Helix(max_offset=30) for _ in range(2)]
+    design = sc.Design(helices=helices, grid=sc.square)
+
+    design.strand(0,0).move(10).cross(1).move(-10).as_circular()
+    design.strand(0,10).move(10).loopout(1, 5).move(-10).as_circular()
+    design.strand(0,20).move(10).cross(1).move(-10)
+
+    return design
+
+
+if __name__ == '__main__':
+    design = create_design()
+    design.write_scadnano_file(directory='output_designs')
diff --git a/examples/output_designs/circular.sc b/examples/output_designs/circular.sc
new file mode 100644
index 00000000..82f0bd44
--- /dev/null
+++ b/examples/output_designs/circular.sc
@@ -0,0 +1,34 @@
+{
+  "version": "0.13.0",
+  "grid": "square",
+  "helices": [
+    {"grid_position": [0, 0]},
+    {"grid_position": [0, 1]}
+  ],
+  "strands": [
+    {
+      "circular": true,
+      "color": "#f74308",
+      "domains": [
+        {"helix": 0, "forward": true, "start": 0, "end": 10},
+        {"helix": 1, "forward": false, "start": 0, "end": 10}
+      ]
+    },
+    {
+      "circular": true,
+      "color": "#57bb00",
+      "domains": [
+        {"helix": 0, "forward": true, "start": 10, "end": 20},
+        {"loopout": 5},
+        {"helix": 1, "forward": false, "start": 10, "end": 20}
+      ]
+    },
+    {
+      "color": "#888888",
+      "domains": [
+        {"helix": 0, "forward": true, "start": 20, "end": 30},
+        {"helix": 1, "forward": false, "start": 20, "end": 30}
+      ]
+    }
+  ]
+}
\ No newline at end of file
diff --git a/scadnano/scadnano.py b/scadnano/scadnano.py
index a0b0459c..79fa077d 100644
--- a/scadnano/scadnano.py
+++ b/scadnano/scadnano.py
@@ -749,6 +749,7 @@ def m13(rotation: int = 5587, variant: M13Variant = M13Variant.p7249) -> str:
 
 # Strand keys
 strand_name_key = 'name'
+circular_key = 'circular'
 color_key = 'color'
 dna_sequence_key = 'sequence'
 legacy_dna_sequence_keys = ['dna_sequence']  # support legacy names for these ideas
@@ -2201,6 +2202,17 @@ def update_to(self, offset: int) -> 'StrandBuilder[StrandLabel, DomainLabel]':
 
         return self
 
+    def as_circular(self) -> 'StrandBuilder[StrandLabel, DomainLabel]':
+        """
+        Makes :any:`Strand` being built circular.
+
+        :return: self
+        """
+        if self._strand is None:
+            raise ValueError('no Strand created yet; make at least one domain first')
+        self._strand.set_circular()
+        return self
+
     # remove quotes when Py3.6 support dropped
     def as_scaffold(self) -> 'StrandBuilder[StrandLabel, DomainLabel]':
         """
@@ -2477,6 +2489,13 @@ class Strand(_JSONSerializable, Generic[StrandLabel, DomainLabel]):
     and could be either single-stranded or double-stranded, 
     whereas each :any:`Loopout` is single-stranded and has no associated :any:`Helix`."""
 
+    circular: bool = False
+    """If True, this :any:`Strand` is circular and has no 5' or 3' end. Although there is still a 
+    first and last :any:`Domain`, we interpret there to be a crossover from the 3' end of the last domain
+    to the 5' end of the first domain, and any circular permutation of :py:data:`Strand.domains` 
+    should result in a functionally equivalent :any:`Strand`. It is illegal to have a 
+    :any:`Modification5Prime` or :any:`Modification3Prime` on a circular :any:`Strand`."""
+
     dna_sequence: Optional[str] = None
     """Do not assign directly to this field. Always use :any:`Design.assign_dna` 
     (for complementarity checking) or :any:`Strand.set_dna_sequence` 
@@ -2509,10 +2528,14 @@ class Strand(_JSONSerializable, Generic[StrandLabel, DomainLabel]):
     :any:`Design` is a scaffold, then the design is considered a DNA origami design."""
 
     modification_5p: Optional[Modification5Prime] = None
-    """5' modification; None if there is no 5' modification."""
+    """
+    5' modification; None if there is no 5' modification. Illegal to have if :py:`Strand.circular` is True.
+    """
 
     modification_3p: Optional[Modification3Prime] = None
-    """3' modification; None if there is no 5' modification."""
+    """
+    3' modification; None if there is no 3' modification. Illegal to have if :py:`Strand.circular` is True.
+    """
 
     modifications_int: Dict[int, ModificationInternal] = field(default_factory=dict)
     """:any:`Modification`'s to the DNA sequence (e.g., biotin, Cy3/Cy5 fluorphores). Maps offset to 
@@ -2574,6 +2597,8 @@ def to_json_serializable(self, suppress_indent: bool = True, **kwargs: Any) -> D
         dct: Dict[str, Any] = OrderedDict()
         if self.name is not None:
             dct[strand_name_key] = self.name
+        if self.circular:
+            dct[circular_key] = self.circular
         if self.color is not None:
             dct[color_key] = self.color.to_json_serializable(suppress_indent)
         if self.dna_sequence is not None:
@@ -2617,6 +2642,7 @@ def from_json(json_map: dict) -> 'Strand':  # remove quotes when Py3.6 support d
             raise IllegalDesignError('Loopout at end of Strand not supported')
 
         is_scaffold = json_map.get(is_scaffold_key, False)
+        circular = json_map.get(circular_key, False)
 
         dna_sequence = optional_field(None, json_map, dna_sequence_key, *legacy_dna_sequence_keys)
 
@@ -2638,6 +2664,7 @@ def decimal_int_to_hex(d: int) -> str:
         return Strand(
             domains=domains,
             dna_sequence=dna_sequence,
+            circular=circular,
             color=color,
             idt=idt,
             is_scaffold=is_scaffold,
@@ -2699,6 +2726,28 @@ def set_color(self, color: Color) -> None:
         """Sets color of this :any:`Strand`."""
         self.color = color
 
+    def set_circular(self, circular: bool = True) -> None:
+        """
+        Sets this to be a circular :any:`Strand` (or non-circular if optional parameter is False.
+
+        :param circular:
+            whether to make this :any:`Strand` circular (True) or linear (False)
+        :raises StrandError:
+            if this :any:`Strand` has a 5' or 3' modification
+        """
+        if circular and self.modification_5p is not None:
+            raise StrandError(self, "cannot have a 5' modification on a circular strand")
+        if circular and self.modification_3p is not None:
+            raise StrandError(self, "cannot have a 3' modification on a circular strand")
+        self.circular = circular
+
+    def set_linear(self) -> None:
+        """
+        Makes this a linear (non-circular) :any:`Strand`. Equivalent to calling
+        `self.set_circular(False)`.
+        """
+        self.set_circular(False)
+
     def set_default_idt(self, use_default_idt: bool = True, skip_scaffold: bool = True,
                         unique_names: bool = False) -> None:
         """
@@ -2744,11 +2793,15 @@ def set_default_idt(self, use_default_idt: bool = True, skip_scaffold: bool = Tr
             self.idt = None
 
     def set_modification_5p(self, mod: Modification5Prime = None) -> None:
-        """Sets 5' modification to be `mod`."""
+        """Sets 5' modification to be `mod`. `mod` cannot be non-None if :any:`Strand.circular` is True."""
+        if self.circular and mod is not None:
+            raise StrandError(self, "cannot have a 5' modification on a circular strand")
         self.modification_5p = mod
 
     def set_modification_3p(self, mod: Modification3Prime = None) -> None:
-        """Sets 3' modification to be `mod`."""
+        """Sets 3' modification to be `mod`. `mod` cannot be non-None if :any:`Strand.circular` is True."""
+        if self.circular and mod is not None:
+            raise StrandError(self, "cannot have a 3' modification on a circular strand")
         self.modification_3p = mod
 
     def remove_modification_5p(self) -> None:
diff --git a/tests/scadnano_tests.py b/tests/scadnano_tests.py
index fc584bd0..bf58fc15 100644
--- a/tests/scadnano_tests.py
+++ b/tests/scadnano_tests.py
@@ -3601,6 +3601,46 @@ def set_colors_black(*strands) -> None:
         strand.set_color(sc.Color(r=0, g=0, b=0))
 
 
+class TestCircularStrands(unittest.TestCase):
+
+    def setUp(self) -> None:
+        helices = [sc.Helix(max_offset=10) for _ in range(2)]
+        self.design = sc.Design(helices=helices, strands=[])
+        self.design.strand(0, 0).move(10).cross(1).move(-10)
+        self.strand = self.design.strands[0]
+        r'''
+        0  [--------\
+                    |
+        1  <--------/
+        '''
+
+    def test_can_add_internal_mod_to_circular_strand(self) -> None:
+        self.strand.set_circular()
+        self.assertEqual(True, self.strand.circular)
+        self.strand.set_modification_internal(2, mod.biotin_int)
+        self.assertEqual(1, len(self.strand.modifications_int))
+
+    def test_cannot_make_strand_circular_if_5p_mod(self) -> None:
+        self.strand.set_modification_5p(mod.biotin_5p)
+        with self.assertRaises(sc.StrandError):
+            self.strand.set_circular(True)
+
+    def test_cannot_make_strand_circular_if_3p_mod(self) -> None:
+        self.strand.set_modification_3p(mod.biotin_3p)
+        with self.assertRaises(sc.StrandError):
+            self.strand.set_circular(True)
+
+    def test_add_5p_mod_to_circular_strand(self) -> None:
+        self.strand.set_circular(True)
+        with self.assertRaises(sc.StrandError):
+            self.strand.set_modification_5p(mod.biotin_5p)
+
+    def test_add_3p_mod_to_circular_strand(self) -> None:
+        self.strand.set_circular(True)
+        with self.assertRaises(sc.StrandError):
+            self.strand.set_modification_3p(mod.biotin_3p)
+
+
 class TestAddStrand(unittest.TestCase):
 
     def test_add_strand__with_loopout(self) -> None: