diff --git a/paramiko/channel.py b/paramiko/channel.py
index 3c9fa35..db85aac 100644
--- a/paramiko/channel.py
+++ b/paramiko/channel.py
@@ -88,6 +88,9 @@ class Channel (object):
         self.combine_stderr = False
         self.exit_status = -1
     
+    def __del__(self):
+        self.close()
+        
     def __repr__(self):
         """
         Return a string representation of this object, for debugging.
@@ -455,11 +458,7 @@ class Channel (object):
         Close the channel.  All future read/write operations on the channel
         will fail.  The remote end will receive no more data (after queued data
         is flushed).  Channels are automatically closed when their L{Transport}
-        is closed.
-        
-        Note that because of peculiarities with the way python's garbage
-        collection works on cycles, channels will B{not} be automatically
-        closed by the garbage collector.
+        is closed or when they are garbage collected.
         """
         self.lock.acquire()
         try:
diff --git a/paramiko/sftp_client.py b/paramiko/sftp_client.py
index a7afdb0..84e32fe 100644
--- a/paramiko/sftp_client.py
+++ b/paramiko/sftp_client.py
@@ -63,6 +63,9 @@ class SFTPClient (BaseSFTP):
                                           self.sock.get_name() + '.sftp')
             self.ultra_debug = transport.get_hexdump()
         self._send_version()
+    
+    def __del__(self):
+        self.close()
 
     def from_transport(selfclass, t):
         """
diff --git a/paramiko/sftp_file.py b/paramiko/sftp_file.py
index 51c7d07..a359510 100644
--- a/paramiko/sftp_file.py
+++ b/paramiko/sftp_file.py
@@ -41,6 +41,9 @@ class SFTPFile (BufferedFile):
         self.handle = handle
         BufferedFile._set_mode(self, mode, bufsize)
 
+    def __del__(self):
+        self.close()
+        
     def close(self):
         BufferedFile.close(self)
         try:
diff --git a/paramiko/transport.py b/paramiko/transport.py
index 86e33ed..9ad8b45 100644
--- a/paramiko/transport.py
+++ b/paramiko/transport.py
@@ -21,6 +21,7 @@ L{BaseTransport} handles the core SSH2 protocol.
 """
 
 import sys, os, string, threading, socket, struct, time
+import weakref
 
 from common import *
 from ssh_exception import SSHException
@@ -205,7 +206,7 @@ class BaseTransport (threading.Thread):
             sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
             sock.connect((hostname, port))
         # okay, normal socket-ish flow here...
-        threading.Thread.__init__(self, target=self._run)
+        threading.Thread.__init__(self)
         self.randpool = randpool
         self.sock = sock
         # Python < 2.3 doesn't have the settimeout method - RogerB
@@ -229,7 +230,7 @@ class BaseTransport (threading.Thread):
         self.initial_kex_done = False
         self.in_kex = False
         self.lock = threading.Lock()    # synchronization (always higher level than write_lock)
-        self.channels = { }             # (id -> Channel)
+        self.channels = weakref.WeakValueDictionary()   # (id -> Channel)
         self.channel_events = { }       # (id -> Event)
         self.channel_counter = 1
         self.window_size = 65536
@@ -249,6 +250,9 @@ class BaseTransport (threading.Thread):
         self.server_accept_cv = threading.Condition(self.lock)
         self.subsystem_table = { }
 
+    def __del__(self):
+        self.close()
+
     def __repr__(self):
         """
         Returns a string representation of this object, for debugging.
@@ -969,7 +973,12 @@ class BaseTransport (threading.Thread):
             raise SSHException('Unknown client cipher ' + name)
         return self._cipher_info[name]['class'].new(key, self._cipher_info[name]['mode'], iv)
 
-    def _run(self):
+    def run(self):
+        # (use the exposed "run" method, because if we specify a thread target
+        # of a private method, threading.Thread will keep a reference to it
+        # indefinitely, creating a GC cycle and not letting Transport ever be
+        # GC'd.  it's a bug in Thread.)
+        
         # active=True occurs before the thread is launched, to avoid a race
         _active_threads.append(self)
         if self.server_mode:
@@ -1281,6 +1290,7 @@ class BaseTransport (threading.Thread):
         # can also free a bunch of stuff here
         self.local_kex_init = self.remote_kex_init = None
         self.K = None
+        self.kex_engine = None
         if not self.initial_kex_done:
             # this was the first key exchange
             self.initial_kex_done = True