diff -r 9d335a54d728 Lib/test/test_zipfile64.py
--- a/Lib/test/test_zipfile64.py	Fri Sep 05 11:01:43 2014 +0300
+++ b/Lib/test/test_zipfile64.py	Fri Sep 05 14:45:29 2014 +0300
@@ -92,7 +92,7 @@
     def testMoreThan64kFiles(self):
         # This test checks that more than 64k files can be added to an archive,
         # and that the resulting archive can be read properly by ZipFile
-        zipf = zipfile.ZipFile(TESTFN, mode="w", allowZip64=False)
+        zipf = zipfile.ZipFile(TESTFN, mode="w", allowZip64=True)
         zipf.debug = 100
         numfiles = (1 << 16) * 3//2
         for i in range(numfiles):
@@ -105,8 +105,44 @@
         for i in range(numfiles):
             content = zipf2.read("foo%08d" % i).decode('ascii')
             self.assertEqual(content, "%d" % (i**3 % 57))
+        zipf2.close()
+
+    def testMoreThan64kFilesAppend(self):
+        zipf = zipfile.ZipFile(TESTFN, mode="w", allowZip64=False)
+        zipf.debug = 100
+        numfiles = (1 << 16) - 1
+        for i in range(numfiles):
+            zipf.writestr("foo%08d" % i, "%d" % (i**3 % 57))
+        self.assertEqual(len(zipf.namelist()), numfiles)
+        with self.assertRaises(zipfile.LargeZipFile):
+            zipf.writestr("foo%08d" % numfiles, b'')
+        self.assertEqual(len(zipf.namelist()), numfiles)
         zipf.close()
 
+        zipf = zipfile.ZipFile(TESTFN, mode="a", allowZip64=False)
+        zipf.debug = 100
+        self.assertEqual(len(zipf.namelist()), numfiles)
+        with self.assertRaises(zipfile.LargeZipFile):
+            zipf.writestr("foo%08d" % numfiles, b'')
+        self.assertEqual(len(zipf.namelist()), numfiles)
+        zipf.close()
+
+        zipf = zipfile.ZipFile(TESTFN, mode="a", allowZip64=True)
+        zipf.debug = 100
+        self.assertEqual(len(zipf.namelist()), numfiles)
+        numfiles2 = (1 << 16) * 3//2
+        for i in range(numfiles, numfiles2):
+            zipf.writestr("foo%08d" % i, "%d" % (i**3 % 57))
+        self.assertEqual(len(zipf.namelist()), numfiles2)
+        zipf.close()
+
+        zipf2 = zipfile.ZipFile(TESTFN, mode="r")
+        self.assertEqual(len(zipf2.namelist()), numfiles2)
+        for i in range(numfiles2):
+            content = zipf2.read("foo%08d" % i).decode('ascii')
+            self.assertEqual(content, "%d" % (i**3 % 57))
+        zipf2.close()
+
     def tearDown(self):
         support.unlink(TESTFN)
         support.unlink(TESTFN2)
diff -r 9d335a54d728 Lib/zipfile.py
--- a/Lib/zipfile.py	Fri Sep 05 11:01:43 2014 +0300
+++ b/Lib/zipfile.py	Fri Sep 05 14:45:29 2014 +0300
@@ -50,7 +50,7 @@
 
 
 ZIP64_LIMIT = (1 << 31) - 1
-ZIP_FILECOUNT_LIMIT = 1 << 16
+ZIP_FILECOUNT_LIMIT = (1 << 16) - 1
 ZIP_MAX_COMMENT = (1 << 16) - 1
 
 # constants for Zip file compression methods
@@ -1304,11 +1304,13 @@
             raise RuntimeError(
                 "Attempt to write ZIP archive that was already closed")
         _check_compression(zinfo.compress_type)
-        if zinfo.file_size > ZIP64_LIMIT:
-            if not self._allowZip64:
+        if not self._allowZip64:
+            if len(self.filelist) >= ZIP_FILECOUNT_LIMIT:
+                raise LargeZipFile(
+                    "Files count would require ZIP64 extensions")
+            if zinfo.file_size > ZIP64_LIMIT:
                 raise LargeZipFile("Filesize would require ZIP64 extensions")
-        if zinfo.header_offset > ZIP64_LIMIT:
-            if not self._allowZip64:
+            if zinfo.header_offset > ZIP64_LIMIT:
                 raise LargeZipFile(
                     "Zipfile size would require ZIP64 extensions")
 
@@ -1464,10 +1466,8 @@
 
         try:
             if self.mode in ("w", "a") and self._didModify: # write ending records
-                count = 0
                 pos1 = self.fp.tell()
                 for zinfo in self.filelist:         # write central directory
-                    count = count + 1
                     dt = zinfo.date_time
                     dosdate = (dt[0] - 1980) << 9 | dt[1] << 5 | dt[2]
                     dostime = dt[3] << 11 | dt[4] << 5 | (dt[5] // 2)
@@ -1531,13 +1531,23 @@
 
                 pos2 = self.fp.tell()
                 # Write end-of-zip-archive record
-                centDirCount = count
+                centDirCount = len(self.filelist)
                 centDirSize = pos2 - pos1
                 centDirOffset = pos1
-                if (centDirCount >= ZIP_FILECOUNT_LIMIT or
+                if (centDirCount > ZIP_FILECOUNT_LIMIT or
                     centDirOffset > ZIP64_LIMIT or
                     centDirSize > ZIP64_LIMIT):
                     # Need to write the ZIP64 end-of-archive records
+                    if not self._allowZip64:
+                        if centDirCount > ZIP_FILECOUNT_LIMIT:
+                            raise LargeZipFile("Files count "
+                                "would require ZIP64 extensions")
+                        if centDirOffset > ZIP64_LIMIT:
+                            raise LargeZipFile("Central directory offset "
+                                "would require ZIP64 extensions")
+                        if centDirSize > ZIP64_LIMIT:
+                            raise LargeZipFile("Central directory size "
+                                "would require ZIP64 extensions")
                     zip64endrec = struct.pack(
                         structEndArchive64, stringEndArchive64,
                         44, 45, 45, 0, 0, centDirCount, centDirCount,