11-Deoxyfistularin-3 (1), a new bromotyrosine derivative, was isolated among other known compounds such as fistularin-3 (2), aerothionin (3), and 11-oxoaerothionin (4) from the Caribbean sponge Aplysinafistularis (Aplysinellidae). The structure of 1 was determined by spectroscopic analysis and showed in vitro activity against the human breast carcinoma cell line MCF-7.