A new cancer cell growth inhibitory (P388 lymphocytic leukemia ED(50) 2.8 microg/mL) cyclodecapeptide designated phakellistatin 12 (2) has been isolated as a trace (1.7 x 10(-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 (500 MHz) 1H, 13C 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.