One new biscembranoid, petronigrione (1), and five known compounds, methyl tortuoate B (2), lobophytone U (3), lobophytone H (4), (24S)-ergostane 3beta, 5alpha, 6beta, 25 tetraol-25-monoacetate (5), and (24S)-ergostane-1beta, 3beta, 5alpha, 6beta, 25-pentaol-25-monoacetate (6), were isolated from the methanol extract of the marine sponge Petrosia nigricans. Their structures were established on the basis of spectral and chemical evidence. The cytotoxicity of all compounds was evaluated by MTT assay on four human cancer cell lines, HepG2, KB, LU-1, and MCF-7. Compounds 1 and 2 exhibited moderate cytotoxic activities on the four human cancer cell lines with IC50 values ranging of 20.7 - 28.9 microg/mL.