Stomachs of shortfin mako sharks Isurus oxyrinchus caught in the northeastern Atlantic by Iberian longliners were analyzed. A number of juveniles, 6 out of 96 individuals with non-empty stomachs, had consumed marine mammals. The remains (skin, fat, vertebrae and flesh with the dorsal fin) were not identifiable at species level by non-genetic methods. Portions of the mitochondrial DNA control region and of the gene coding for cytochrome b were therefore sequenced. Both the short-beaked common dolphin Delphinus delphis and possibly the striped dolphin Stenella coeruleoalba were identified. Shortfin makos are able to consume marine mammals almost as large as themselves. Well-preserved D. delphis were juveniles.