The wavy turban snail, Megastraea undosa (W. Wood), is an important fishery resource along the Mexican Pacific coast and a keystone species in subtropical rocky reefs. Its diet was determined from stomach contents of 125 specimens collected in three rocky reefs of the western coast of the Baja California Peninsula in July and November 2006 and March 2007. The snail consumed 20 taxa of seaweeds, 1 seagrass, and 11 taxa of invertebrates. The diet had significant variation depending on site and date. Main food items were Macrocystis pyrifera, Ecklonia arborea, and Corallina spp. Of secondary importance were red algae of the family Delesseriaceae and the genera Plocamium and Gelidium, the brown alga Stephanocystis osmundacea, the seagrasses Phyllospadix spp., the hydrozoan Dynamena, and the isopod Idotea. Analysis showed that the snail was a grazer with a mixed feeding strategy, feeding abundantly on kelp or coralline algae and also consuming many other resources, which was reflected in its variable trophic niche width with a Levins index ranging from 0.21 to 0.79.