An investigation of a sponge rich in psammaplin derivatives provides some insight into the biogenesis of this group of amino acid derivatives. Accompanying psammaplin A (1) were a new cysteine dimer, prepsammaplin A (2), and three new bromotyrosine-cysteine derivatives, psammaplins B (3), C (4), and D (5). Two other compounds were isolated including (3-bromo4hydroxy) benxaldehyde (6) and (3-bromo4hydroxyphenyl)~acetonitrile (7) which appear to be bromotytosine derivatives. Psammaplln D (5) showed antimicrobial and mild tymsine khrase inhibition activity.