diff --git a/pyrsistent/_helpers.py b/pyrsistent/_helpers.py
index 1320e65..b44bfc5 100644
--- a/pyrsistent/_helpers.py
+++ b/pyrsistent/_helpers.py
@@ -1,3 +1,4 @@
+import collections
 from functools import wraps
 from pyrsistent._pmap import PMap, pmap
 from pyrsistent._pset import PSet, pset
@@ -10,6 +11,7 @@ def freeze(o, strict=True):
 
     - list is converted to pvector, recursively
     - dict is converted to pmap, recursively on values (but not keys)
+    - defaultdict is converted to pmap, recursively on values (but not keys)
     - set is converted to pset, but not recursively
     - tuple is converted to tuple, recursively.
 
@@ -33,6 +35,8 @@ def freeze(o, strict=True):
     typ = type(o)
     if typ is dict or (strict and isinstance(o, PMap)):
         return pmap({k: freeze(v, strict) for k, v in o.items()})
+    if typ is collections.defaultdict or (strict and isinstance(o, PMap)):
+        return pmap({k: freeze(v, strict) for k, v in o.items()})
     if typ is list or (strict and isinstance(o, PVector)):
         curried_freeze = lambda x: freeze(x, strict)
         return pvector(map(curried_freeze, o))

diff --git a/tests/freeze_test.py b/tests/freeze_test.py
index 6e0e2eb..158cf5d 100644
--- a/tests/freeze_test.py
+++ b/tests/freeze_test.py
@@ -1,5 +1,5 @@
 """Tests for freeze and thaw."""
-
+import collections
 from pyrsistent import v, m, s, freeze, thaw, PRecord, field, mutant
 
 
@@ -17,6 +17,13 @@ def test_freeze_dict():
     assert result == m(a='b')
     assert type(freeze({'a': 'b'})) is type(m())
 
+def test_freeze_defaultdict():
+    test_dict = collections.defaultdict(dict)
+    test_dict['a'] = 'b'
+    result = freeze(test_dict)
+    assert result == m(a='b')
+    assert type(freeze({'a': 'b'})) is type(m())
+
 def test_freeze_set():
     result = freeze(set([1, 2, 3]))
     assert result == s(1, 2, 3)
@@ -27,6 +34,13 @@ def test_freeze_recurse_in_dictionary_values():
     assert result == m(a=v(1))
     assert type(result['a']) is type(v())
 
+def test_freeze_recurse_in_defaultdict_values():
+    test_dict = collections.defaultdict(dict)
+    test_dict['a'] = [1]
+    result = freeze(test_dict)
+    assert result == m(a=v(1))
+    assert type(result['a']) is type(v())
+
 def test_freeze_recurse_in_pmap_values():
     input = {'a': m(b={'c': 1})}
     result = freeze(input)

freeze() does not work on collections.defaultdict
The following code

```py
import collections
from pyrsistent import freeze

mydict = collections.defaultdict(dict)
mydict["a"]["x"] = 2
mydict["a"]["y"] = 3
mydict["b"]["z"] = 4

frozen = freeze(mydict)
print(type(frozen))
```

prints `<class 'collections.defaultdict'>` rather than `PMap` as I would expect. In other words `freeze` does not do anything to a `collections.defaultdict` when I would expect it to make it a persistent map instead.
tests/freeze_test.py::test_freeze_recurse_in_defaultdict_values
tests/freeze_test.py::test_freeze_basic tests/freeze_test.py::test_freeze_list tests/freeze_test.py::test_freeze_dict tests/freeze_test.py::test_freeze_defaultdict tests/freeze_test.py::test_freeze_set tests/freeze_test.py::test_freeze_recurse_in_dictionary_values tests/freeze_test.py::test_freeze_recurse_in_pmap_values tests/freeze_test.py::test_freeze_recurse_in_lists tests/freeze_test.py::test_freeze_recurse_in_pvectors tests/freeze_test.py::test_freeze_recurse_in_tuples tests/freeze_test.py::test_freeze_nonstrict_no_recurse_in_pmap_values tests/freeze_test.py::test_freeze_nonstrict_no_recurse_in_pvectors tests/freeze_test.py::test_thaw_basic tests/freeze_test.py::test_thaw_list tests/freeze_test.py::test_thaw_dict tests/freeze_test.py::test_thaw_set tests/freeze_test.py::test_thaw_recurse_in_mapping_values tests/freeze_test.py::test_thaw_recurse_in_dict_values tests/freeze_test.py::test_thaw_recurse_in_vectors tests/freeze_test.py::test_thaw_recurse_in_lists tests/freeze_test.py::test_thaw_recurse_in_tuples tests/freeze_test.py::test_thaw_can_handle_subclasses_of_persistent_base_types tests/freeze_test.py::test_thaw_non_strict_no_recurse_in_dict_values tests/freeze_test.py::test_thaw_non_strict_no_recurse_in_lists tests/freeze_test.py::test_mutant_decorator