Due to unprecedented demand, Bittrex will be opening up an Ethereum Classic (ETC) market. Bittrex users will be credited the amount of ETC that they had at the time of the hard fork and be able to generate a separate Ethereum Classic deposit address. Note that due to the current replay attacks occurring against both chains and potential of future cross chain attacks, Bittrex WILL NOT be crediting you if you deposit ETC to an ETH deposit address or vice versa under any circumstance. Please make sure you are depositing on the proper chain. We are doing this to ensure separation of chains and not cross contaminate key material. We are encouraging anyone who is using ETC to use a split contract to ensure that their coins are not vulnerable to replaying. A message linking to this post has been displayed in both your ETH and ETC deposit windows. By depositing ETC or ETH to your Bittrex address, you are agreeing to the aforementioned terms.
I had Ethereum at the time of the fork, why don't I have Ethereum classic in my Bittrex account?
You were not credited Ethereum classic if you withdrew it from your Bittrex account between the time of the hardfork and now. You will be able to track this via the Ethereum classic block explorer http://gastracker.io/.
What is the replay attack and should I take precautions?
Yes! See this article for more details: http://vessenes.com/do-not-mess-with-eth-classic-it-will-f-you-up/
What does the Ethereum Foundation have to say?