Two new aromatic alkaloids, trididemnic acids A (1) and B (2), have been isolated from the marine ascidian Trididemnum sp. The structures of 1 and 2 were solved by spectroscopic analysis of their permethylated derivatives 4 and 5. Single crystal x-ray diffraction analysis of 4 confirmed the structures. An interesting retro-Fries rearrangement reaction, involving the conversion of 3 to 8 under bromination conditions, was also discovered.