New glycosphingolipids, agelasphins (1-4), were isolated from the marine sponge Agelas mauritianus, and their structures were determined. In our quest for bioactive substances from marine organisms, we found these novel glycosphingolipids possessing an α-galactosylceramide structure from an extract of the sponge Agelas mauritianus Carter (1883). To the best of our knowledge, this is the first report on the isolation of α-galactosylceramides from natural sources. We herein describe the isolation and the structure elucidation of these novel substances, named agelasphins (AGLs). Although a number of β-galactosylceramides have been known, AGLs are the first cerebrosides having an α-galactosyl linkage. All AGLs showed antitumor activity.