Thirteen new polyhydroxysterols (1-5, 7-12, 14, 15) and two known polyhydroxysterols (6, 13) were isolated from the brine shrimp active fraction of the starfish Certonardoa semiregularis. The structures were determined on the basis of spectral analysis and chemical derivatization. These compounds displayed considerable cytotoxicity against a small panel of human solid tumor cell lines. These compounds were also evaluated for antibacterial activity against 20 clinically isolated strains. Most of the compounds showed weak antibacterial activity against Streptococcus pyogenes 308A, Pseudomonas aeruginosa 1771, and Pseudomonas aeruginosa 1771M.