Accused child murderer Casey Anthony is in more hot water with the feds -- the I.R.S. believes she failed to pay $68k in taxes back in 2008 ... the same year she allegedly killed her 2-year-old daughter.



TMZ obtained a tax lien filed against Anthony in Florida last week -- and according to the document, Casey never forked over $68,520 in federal taxes in 2008.

Casey was charged with first-degree murder the same year -- and is currently on trial in Orlando, FL ... facing the death penalty if convicted.

Calls to Casey's attorney were not immediately returned.