A novel sesquiterpenoid (1) and three known compounds identified as isoaltenuene (2), altenuene (3), and alternariol 4, 10-O-dimethyl ether (4), were isolated in our investigation of the cytotoxic constituents from solid cultures of the endophytic fungus Colletotrichum sp. The structures of these compounds were elucidated through spectroscopic data analysis. All compounds exhibited cytotoxic activity against lung cancer cell line A549, breast cancer cell line MDA-MB-231 and pancreatic cancer cell line PANC-1. Compound 4 could induce the PANC-1 cells inflation or death, but couldn't induce apoptosis at the IC50 of 60.2 microg x mL(-1).