The anaspidean mollusk Aplysia fasciata fromRío San Pedro (Cádiz, Spain) contains theknowndegraded sterols aplykurodinone B (2) and aplykurodin B(4) together with the new 3-epi-aplykurodinone B (5). The structure of 5 waselucidated by interpretation of spectral data,and its relative stereochemistry was established using NOEDSexperiments. The newcompound 5 exhibited cytotoxicity against four tumor celllines.