Recent chemical studies of colonial tunicates have shown them to be rich sources of fused tetra- and pentacyclic aromatic alkaloids. In our earlier work on Trididemnum sp., a purple colonial tunicate from Pago Bay, Guam, we isolated a thiazinone-containing pentacyclic alkaloid, shermilamine A (1). Further fractionation of the extracts has yielded a second shermilamine alkaloid, shermilamine B (2), lacking C6 bromination. The structure of shermilamine B was determined by interpretation of spectral data. Furthermore, a ¹H-detected heteronuclear multiple bond ¹H-¹³C correlation experiment (HMBC) has facilitated the complete and unambiguous assignment of ¹H and ¹³C resonances for both shermilamine A and B. As a result of this experiment, a number of the spectral properties previously reported for shermilamine A have been revised.