Pretty much in agreement with James K and Pam Shoup, although less optimistic than she that was addressed in a document. Unless the plan document, the stock purchase agreement, or some other corporate instrument appoints someone else as plan administrator, the default "administrator" with legal responsibility to file the 5500's is the current sponsoring employer. This becomes a routine business expense of the sub that the acquirer bought into when it bought the sub, like unpaid bills or fines. Place to deal with this was probably the stock purchase agreement. You should probably check to make sure it wasn't addressed there.