diff --git a/src/dawg.pyx b/src/dawg.pyx
index 133c7fc..de0d7f5 100644
--- a/src/dawg.pyx
+++ b/src/dawg.pyx
@@ -86,7 +86,7 @@ cdef class DAWG:
     cpdef bint b_has_key(self, bytes key) except -1:
         return self.dct.Contains(key, len(key))
 
-    cpdef bytes tobytes(self):
+    cpdef bytes tobytes(self) except +:
         """
         Return raw DAWG content as bytes.
         """
@@ -249,6 +249,24 @@ cdef class DAWG:
 
         return res
 
+    def longest_prefix(self, unicode key):
+        cdef BaseType index = self.dct.root()
+        cdef int pos = 1
+        cdef int lastpos = 0
+        cdef CharType ch
+
+        for ch in key:
+            if not self.dct.Follow(ch, &index):
+                break
+            if self.dct.has_value(index):
+                lastpos = pos
+            pos += 1
+
+        if lastpos:
+            return key[:lastpos]
+        else:
+            raise KeyError("No prefix found")
+
     def iterprefixes(self, unicode key):
         '''
         Return a generator with keys of this DAWG that are prefixes of the ``key``.
@@ -798,7 +816,28 @@ cdef class BytesDAWG(CompletionDAWG):
         """
         return self._similar_item_values(0, key, self.dct.root(), replaces)
 
+    def longest_prefix(self, unicode key):
+        cdef BaseType index = self.dct.root()
+        cdef BaseType tmp
+        cdef BaseType lastindex
+        cdef int pos = 1
+        cdef int lastpos = 0
+        cdef CharType ch
+
+        for ch in key:
+            if not self.dct.Follow(ch, &index):
+                break
+
+            tmp = index
+            if self.dct.Follow(self._c_payload_separator, &tmp):
+                lastpos = pos
+                lastindex = tmp
+            pos += 1
 
+        if lastpos:
+            return key[:lastpos], self._value_for_index(lastindex)
+        else:
+            raise KeyError("No prefix found")
 
 cdef class RecordDAWG(BytesDAWG):
     """
@@ -900,6 +939,26 @@ cdef class IntDAWG(DAWG):
     cpdef int b_get_value(self, bytes key):
         return self.dct.Find(key)
 
+    def longest_prefix(self, unicode key):
+        cdef BaseType index = self.dct.root()
+        cdef BaseType lastindex
+        cdef int pos = 1
+        cdef int lastpos = 0
+        cdef CharType ch
+
+        for ch in key:
+            if not self.dct.Follow(ch, &index):
+                break
+
+            if self.dct.has_value(index):
+                lastpos = pos
+                lastindex = index
+            pos += 1
+
+        if lastpos:
+            return key[:lastpos], self.dct.value(lastindex)
+        else:
+            raise KeyError("No prefix found")
 
 # FIXME: code duplication.
 cdef class IntCompletionDAWG(CompletionDAWG):
diff --git a/tests/test_dawg.py b/tests/test_dawg.py
index 26f6627..92baa4b 100644
--- a/tests/test_dawg.py
+++ b/tests/test_dawg.py
@@ -83,7 +83,12 @@ def test_unicode_sorting(self):
         # if data is sorted according to unicode rules.
         dawg.DAWG([key1, key2])
 
-
+    def test_longest_prefix(self):
+        d = dawg.DAWG(["a", "as", "asdf"])
+        assert d.longest_prefix("a") == "a"
+        assert d.longest_prefix("as") == "as"
+        assert d.longest_prefix("asd") == "as"
+        assert d.longest_prefix("asdf") == "asdf"
 
 class TestIntDAWG(object):
 
@@ -148,6 +153,13 @@ def test_int_value_ranges(self):
         with pytest.raises(OverflowError):
             self.IntDAWG({'f': 2**32-1})
 
+    def test_longest_prefix(self):
+        d = dawg.IntDAWG([("a", 1), ("as", 2), ("asdf", 3)])
+        assert d.longest_prefix("a") == ("a", 1)
+        assert d.longest_prefix("as") == ("as", 2)
+        assert d.longest_prefix("asd") == ("as", 2)
+        assert d.longest_prefix("asdf") == ("asdf", 3)
+
 
 class TestIntCompletionDAWG(TestIntDAWG):
     IntDAWG = dawg.IntCompletionDAWG  # checks that all tests for IntDAWG pass
diff --git a/tests/test_payload_dawg.py b/tests/test_payload_dawg.py
index 305ac3f..a8b2b1b 100644
--- a/tests/test_payload_dawg.py
+++ b/tests/test_payload_dawg.py
@@ -83,7 +83,12 @@ def test_build_error(self):
         with pytest.raises(dawg.Error):
             self.dawg(payload_separator=b'f')
 
-
+    def test_longest_prefix(self):
+        d = dawg.BytesDAWG([("a", b"a1"), ("a", b"a2"), ("as", b"as"), ("asdf", b"asdf")])
+        assert d.longest_prefix("a") == ("a", [b"a1", b"a2"])
+        assert d.longest_prefix("as") == ("as", [b"as"])
+        assert d.longest_prefix("asd") == ("as", [b"as"])
+        assert d.longest_prefix("asdf") == ("asdf", [b"asdf"])
 
 class TestRecordDAWG(object):