Two new cytotoxic dioxopiperazine dimers, 11,11′-dideoxyverticillin A (1) and 11′-deoxyverticillin A (2), and the previously described verticillin A (3), have been isolated from the mycelium of a marine-derived fungus of the genusPenicillium. an inactive, but newbisdethio-bis(methylthio)-dioxopiperazine, 4, was also obtained by extraction of the culture broth. the structures and absolute stereochemistries of the new compounds were assigned on the basis of NMR and CD experiments. Compounds 1 and 2 exhibit potentin vitrocytotoxicity against HCT-116 human colon carcinoma (IC50= 30ng/ml).