diff -r 7e8ac22a4cf2 Lib/test/test_bisect.py
--- a/Lib/test/test_bisect.py	Sat Mar 10 09:27:30 2012 +0100
+++ b/Lib/test/test_bisect.py	Sat Mar 10 15:27:35 2012 +0000
@@ -122,6 +122,13 @@
         self.assertRaises(ValueError, mod.insort_left, [1, 2, 3], 5, -1, 3),
         self.assertRaises(ValueError, mod.insort_right, [1, 2, 3], 5, -1, 3),
 
+    def test_large_range(self):
+        # Issue 13496
+        mod = self.module
+        data = range(sys.maxsize-1)
+        self.assertEqual(mod.bisect_left(data, sys.maxsize-3), sys.maxsize-3)
+        self.assertEqual(mod.bisect_right(data, sys.maxsize-3), sys.maxsize-2)
+
     def test_random(self, n=25):
         from random import randrange
         for i in range(n):
diff -r 7e8ac22a4cf2 Modules/_bisectmodule.c
--- a/Modules/_bisectmodule.c	Sat Mar 10 09:27:30 2012 +0100
+++ b/Modules/_bisectmodule.c	Sat Mar 10 15:27:35 2012 +0000
@@ -21,7 +21,10 @@
             return -1;
     }
     while (lo < hi) {
-        mid = (lo + hi) / 2;
+        /* The (size_t)cast ensures that the addition and subsequent division
+           are performed as unsigned operations, avoiding difficulties from
+           signed overflow.  (See issue 13496.) */
+        mid = ((size_t)lo + hi) / 2;
         litem = PySequence_GetItem(list, mid);
         if (litem == NULL)
             return -1;
@@ -123,7 +126,10 @@
             return -1;
     }
     while (lo < hi) {
-        mid = (lo + hi) / 2;
+        /* The (size_t)cast ensures that the addition and subsequent division
+           are performed as unsigned operations, avoiding difficulties from
+           signed overflow.  (See issue 13496.) */
+        mid = ((size_t)lo + hi) / 2;
         litem = PySequence_GetItem(list, mid);
         if (litem == NULL)
             return -1;