The lithistid sponge Scleritoderma nodosum contains a new cyclic peptide, scleritodermin A (1), the structure of which incorporates l-proline, l-serine, and keto-allo-isoleucine units as well as a novel conjugated thiazole moiety (ACT) and O-methyl-N-sulfoserine. Scleritodermin A (1) inhibited tubulin polymerization and showed significant in vitro cytotoxicity against human tumor cell lines.