diff --git a/casimport/__init__.py b/casimport/__init__.py
index 36515b8..5098480 100644
--- a/casimport/__init__.py
+++ b/casimport/__init__.py
@@ -398,6 +398,11 @@ class CASFinder(MetaPathFinder, Loader):
 			else:
 				raise ValueError('unable to find loader for url %s' % repr(urllib.parse.urlunparse(url)))
 
+		hash = hashlib.sha256(data).hexdigest()
+		wrtr = self._writer
+		if wrtr is not None:
+			wrtr.write_cache(hash, data)
+
 		exec(data, module.__dict__)
 
 		# we were successful, add the aliases
@@ -767,7 +772,7 @@ class Test(unittest.TestCase):
 		sys.path.append(fixdir)
 
 		# that a fake ipfsloader
-		with open(self.fixtures / 'hello.py') as fp:
+		with open(self.fixtures / 'hello.py', 'rb') as fp:
 			# that returns the correct data
 			fakedata = fp.read()
 
@@ -784,9 +789,20 @@ class Test(unittest.TestCase):
 			with CASFinder() as f, \
 			    tempattrset(sys.modules[__name__],
 			    'load_mod_aliases', f.load_mod_aliases):
+				# that a cache dir
+				cachedir = self.tempdir / 'cache'
+				fd = FileDirCAS(cachedir)
 
+				# when registered
+				f.register(fd)
+
+				# as a writer
+				f.register_write(fd)
+
+				# and the fake ipfs module
 				f.register(fakeipfsloader)
 
+				# and the pkg we are about to load, isn't
 				self.assertNotIn('randpkg', sys.modules)
 
 				# that the import is successful
@@ -795,6 +811,12 @@ class Test(unittest.TestCase):
 				# and pulled in the method
 				self.assertTrue(hasattr(randpkg, 'hello'))
 
+				# that the cache was written to, and matches
+				hash = hashlib.sha256(fakedata).hexdigest()
+				filecmp.cmp(cachedir / '3' / (hash + '.py'),
+				    self.fixtures / 'hello.py')
+
+				# clean up
 				del sys.modules['randpkg']
 		finally:
 			sys.path.remove(fixdir)