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Do I need to report to FInCEN if I had greater than $10,000 worth of bitcoin in a foreign bitcoin exchange?
Firstly you have to know exactly what you are asking here.  What you have if you "own" bitcoins is a private key that allows you to make a change to the blockchain that can assign a piece of information from yourself to the next person. Nothing more nothing less.  The fact that this small piece of information is considered to have a market value, is a matter of opinion, and is analagous to owning a domain name. A domain name is an entry in a register, that has equal weight to all other entries, but the market determines if that information (eg: CocaCola.com) has any more value than say another less well know domain. Bitcoin is the same - an entry in a register, and the market decides which entry is more valuable than another.  So what exactly are you wanting to declare to FinCEN? Are you willing to declare the ownership of private key? Of course not. So what then? An uncrackable private key can be generated at will by anyone, without even needing to "own" or transact in bitcoins, and that same private key would be equally valid on any of the 1000's of other bitcoin clones. The point I want to make is that owning a private key in itself is not valuable. Therefore you do not need, nor would anyone advise notifying FinCEN of that fact. To put this into context, every time you connect to online banking, your computer secretly generates a new random private key to secure your communications with the bank. Theoretically that same private key could also be used to sign a bitcoin transaction. Do you need to declare every private key your computer generates? No. Secondly, if you are using any of the latest generation of HD wallets, your private key changes with every single transaction. Are you seriously saying that you want to take it on your shoulders to inform FinCEN every time you move information (bitcoin amounts) around even in your own wallets? The fact is FinCEN could never "discover" your ownership of bitcoins (or any of the 1000s of alt coins) other than by you informing them of this fact.  You may want to carefully consider the personal implications of starting down this road especially as all FinCEN would need to do is subpoena your bitcoin private key to steal your so-called funds, as they have done recently to other more prominent persons in the community. EDIT to clarify the points raised in comments. You do not own the private key to the bitcoins stored on a foreign exchange, nor can you discover it. The exchange owns the private key. You therefore do not either technically have control over the coins (MtGox is a very good example here - they went out of business because they allowed their private keys to be used by some other party who was able to siphon off the coins). Your balance is only yours when you own the private keys and the ability to spend. Any other situation you can neither recover the bitcoin to sell (to pay for any taxes due). So you do not either have the legal right nor the technical right to consider those coins in your possession.  For those who do not understand the technical or legal implications of private key ownership, please do not speculate about what "owning" bitcoin actually means, or how ownership can be discovered. Holding Bitcoin is not illegal, and the US government who until recently were the single largest holder of Bitcoin demonstrate simply by this fact alone that there is nothing untoward here.