The tunicate Lissoclinum patella produces a family of lipophilic cyclic peptides all of which contain an unusual fused oxazoline-thiazole unit. The structures of three of these peptides, patellamides A-C have been determined by chemical and spectral methods. The patellamides are cytotoxic, exhibiting IC50 values of 2-4 μg/mL against L1210 murine leukemia cells. Additionally, patellamide A was active against the human ALL cell line CEM with an ID50 of 0.028 μg/mL. Ulithiacyclamide, a peptide previously reported from L. patella was also tested for cytotoxicity and exhibited 50% inhibition at doses of 0.35 and 0.01 μg/mL for the L1210 and CEM tests, respectively.