Hi, I am not a US citizen (I am Croatian citizen) but have lived in the US for 10 years (mostly on F1 visa and OPT). I started a full time job in 2009 (on OPT) and I was considered a resident alien for tax purposes. I changed my status last summer from F1 to J1 and starting this year my employer stopped withholding FICA and they said my status has changed from resident to non resident alien for tax purposes. How is that possible? Does it have to do with me changing from F1 to J1? Thanks!

I believe your employer is in error.

It IS possible to go back to non-resident alien status when you switch to a J-1 visa if it is addressed in the tax treaty between the U.S. and your home country, but I see NO tax treaty with Croatia, so normal tax law would apply.

Under normal tax law, you have long used up the exemption period for FICA tax exemption for non-resident aliens. In my opinion, you are liable for FICA taxes, and you have the legal oblifgation to inform your employer of that fact. Do that in writing; email is a godd vehicle for that. If the employer chooses NOT to believe you, then it is no longer your problem.