The sponge Petrosia sp. yielded five polyacetylenic compounds (1-5), including two new polyacetylenes, petrosianynes A (1) and B (2). The structures of these compounds were elucidated by detailed spectroscopic analysis and by comparison with the physical and spectral data of related known analogues. Compounds 1-5 exhibited significant cytotoxic activity against a limited panel of cancer cell lines.