diff --git a/moviepy/audio/io/ffmpeg_audiowriter.py b/moviepy/audio/io/ffmpeg_audiowriter.py
index e895f5d80..81bc88848 100644
--- a/moviepy/audio/io/ffmpeg_audiowriter.py
+++ b/moviepy/audio/io/ffmpeg_audiowriter.py
@@ -2,6 +2,9 @@
 import numpy as np
 import subprocess as sp
 
+import sys
+PY3=sys.version_info.major >= 3
+
 import os
 try:
     from subprocess import DEVNULL # py3k
@@ -82,14 +85,17 @@ def __init__(self, filename, fps_input, nbytes=2,
 
     def write_frames(self,frames_array):
         try:
-            self.proc.stdin.write(frames_array.tostring())
+            if PY3:
+               self.proc.stdin.write(frames_array.tobytes())
+            else:
+               self.proc.stdin.write(frames_array.tostring())
         except IOError as err:
             ffmpeg_error = self.proc.stderr.read()
             error = (str(err)+ ("\n\nMoviePy error: FFMPEG encountered "
                      "the following error while writing file %s:"%self.filename
                      + "\n\n"+ffmpeg_error))
 
-            if "Unknown encoder" in ffmpeg_error:
+            if b"Unknown encoder" in ffmpeg_error:
 
                 error = (error+("\n\nThe audio export failed because "
                     "FFMPEG didn't find the specified codec for audio "
@@ -99,7 +105,7 @@ def write_frames(self,frames_array):
                     "   >>> to_videofile('myvid.mp4', audio_codec='libmp3lame')"
                     )%(self.codec))
 
-            elif "incorrect codec parameters ?" in ffmpeg_error:
+            elif b"incorrect codec parameters ?" in ffmpeg_error:
 
                 error = error+("\n\nThe audio export "
                   "failed, possibly because the codec specified for "
@@ -108,7 +114,7 @@ def write_frames(self,frames_array):
                   "argument in to_videofile. This would be 'libmp3lame' "
                   "for mp3, 'libvorbis' for ogg...")%(self.codec, self.ext)
 
-            elif  "encoder setup failed":
+            elif  b"encoder setup failed" in ffmpeg_error:
 
                 error = error+("\n\nThe audio export "
                   "failed, possily because the bitrate you specified "
diff --git a/moviepy/video/io/ffmpeg_writer.py b/moviepy/video/io/ffmpeg_writer.py
index 605015206..edf1b9fd9 100644
--- a/moviepy/video/io/ffmpeg_writer.py
+++ b/moviepy/video/io/ffmpeg_writer.py
@@ -7,6 +7,10 @@
 import os
 import numpy as np
 
+import sys
+
+PY3 = sys.version_info.major >=3
+
 try:
     from subprocess import DEVNULL  # py3k
 except ImportError:
@@ -134,14 +138,17 @@ def __init__(self, filename, size, fps, codec="libx264", audiofile=None,
     def write_frame(self, img_array):
         """ Writes one frame in the file."""
         try:
-            self.proc.stdin.write(img_array.tostring())
+            if PY3:
+               self.proc.stdin.write(img_array.tobytes())
+            else:
+               self.proc.stdin.write(img_array.tostring())
         except IOError as err:
             ffmpeg_error = self.proc.stderr.read()
             error = (str(err) + ("\n\nMoviePy error: FFMPEG encountered "
                                  "the following error while writing file %s:"
                                  "\n\n %s" % (self.filename, ffmpeg_error)))
 
-            if "Unknown encoder" in ffmpeg_error:
+            if b"Unknown encoder" in ffmpeg_error:
 
                 error = error+("\n\nThe video export "
                   "failed because FFMPEG didn't find the specified "
@@ -150,7 +157,7 @@ def write_frame(self, img_array):
                   "write_videofile. For instance:\n"
                   "  >>> clip.write_videofile('myvid.webm', codec='libvpx')")%(self.codec)
 
-            elif "incorrect codec parameters ?" in ffmpeg_error:
+            elif b"incorrect codec parameters ?" in ffmpeg_error:
 
                  error = error+("\n\nThe video export "
                   "failed, possibly because the codec specified for "
@@ -164,13 +171,13 @@ def write_frame(self, img_array):
                   "video codec."
                   )%(self.codec, self.ext)
 
-            elif  "encoder setup failed" in ffmpeg_error:
+            elif  b"encoder setup failed" in ffmpeg_error:
 
                 error = error+("\n\nThe video export "
                   "failed, possibly because the bitrate you specified "
                   "was too high or too low for the video codec.")
 
-            elif "Invalid encoder type" in ffmpeg_error:
+            elif b"Invalid encoder type" in ffmpeg_error:
 
                 error = error + ("\n\nThe video export failed because the codec "
                   "or file extension you provided is not a video")