A new cancer cell growth inhibitory (P388 lymphocytic leukemia ED 5 0 2.8 μg/mL) cyclodecapeptide designated phakellistatin 12 (2) has been isolated as a trace (1.7x10 - 6 % yield) constituent of the Western Pacific Ocean (Federated States of Micronesia-Chuuk) sponge Phakellia sp. Employing principally a combination of high resolution FAB with high field (500MHz) 1 H, 1 3 C and 2-D NMR and chiral GC analyses the structure (all S chirality) cyclo-Ile-Phe-Thr-Leu-Pro-Pro-Tyr-Ile-Pro-Pro was assigned.