Zygosporins D, E, F, and G were isolated as minor metabolites of the fungus Zygosporium masonii and their cycloundeca[d]isoindole-1,11-dione structures were elucidated as (II), (IV), (III), and (IX), respectively. The cytotoxicities (ED50) against HeLa cells (monolayer culture) of zygosporins D, E, F, and G were 0.79, 2.65, > 10.0, and 4.9 pg./ml., respectively.