diff --git a/paramiko/sftp_client.py b/paramiko/sftp_client.py
index 79a7761..189caa4 100644
--- a/paramiko/sftp_client.py
+++ b/paramiko/sftp_client.py
@@ -682,7 +682,7 @@ class SFTPClient (BaseSFTP):
                     self._convert_status(msg)
                 return t, msg
             if fileobj is not type(None):
-                fileobj._async_response(t, msg)
+                fileobj._async_response(t, msg, num)
             if waitfor is None:
                 # just doing a single check
                 break
diff --git a/paramiko/sftp_file.py b/paramiko/sftp_file.py
index 8c5c7ac..2d4d431 100644
--- a/paramiko/sftp_file.py
+++ b/paramiko/sftp_file.py
@@ -49,7 +49,8 @@ class SFTPFile (BufferedFile):
         self._prefetching = False
         self._prefetch_done = False
         self._prefetch_data = {}
-        self._prefetch_reads = []
+        self._prefetch_extents = {}
+        self._prefetch_lock = threading.Lock()
         self._saved_exception = None
 
     def __del__(self):
@@ -86,7 +87,7 @@ class SFTPFile (BufferedFile):
             pass
 
     def _data_in_prefetch_requests(self, offset, size):
-        k = [i for i in self._prefetch_reads if i[0] <= offset]
+        k = [self._prefetch_extents[i] for i in self._prefetch_extents if self._prefetch_extents[i][0] <= offset]
         if len(k) == 0:
             return False
         k.sort(lambda x, y: cmp(x[0], y[0]))
@@ -440,7 +441,6 @@ class SFTPFile (BufferedFile):
     def _start_prefetch(self, chunks):
         self._prefetching = True
         self._prefetch_done = False
-        self._prefetch_reads.extend(chunks)
 
         t = threading.Thread(target=self._prefetch_thread, args=(chunks,))
         t.setDaemon(True)
@@ -450,9 +450,11 @@ class SFTPFile (BufferedFile):
         # do these read requests in a temporary thread because there may be
         # a lot of them, so it may block.
         for offset, length in chunks:
-            self.sftp._async_request(self, CMD_READ, self.handle, long(offset), int(length))
+            with self._prefetch_lock:
+                num = self.sftp._async_request(self, CMD_READ, self.handle, long(offset), int(length))
+                self._prefetch_extents[num] = (offset, length)
 
-    def _async_response(self, t, msg):
+    def _async_response(self, t, msg, num):
         if t == CMD_STATUS:
             # save exception and re-raise it on next file operation
             try:
@@ -463,10 +465,12 @@ class SFTPFile (BufferedFile):
         if t != CMD_DATA:
             raise SFTPError('Expected data')
         data = msg.get_string()
-        offset, length = self._prefetch_reads.pop(0)
-        self._prefetch_data[offset] = data
-        if len(self._prefetch_reads) == 0:
-            self._prefetch_done = True
+        with self._prefetch_lock:
+            offset,length = self._prefetch_extents[num]
+            self._prefetch_data[offset] = data
+            del self._prefetch_extents[num]
+            if len(self._prefetch_extents) == 0:
+                self._prefetch_done = True
     
     def _check_exception(self):
         "if there's a saved exception, raise & clear it"