Diet of the volcano keyhole limpet Fissurella volcano was determined from stomach contents of 56 specimens collected from two rocky reefs along the west coast of the Baja California Peninsula of Mexico in July and November 2006 and March 2007. We identified 40 taxa, including 27 diatoms, one cyanoprokaryota, eight macroalgae (mainly red algae), one sea grass, and three protozoans. Food items >20% in relative frequency were diatoms, cyanoprokaryota, and Pyropia sp. Among the diatoms, 10 species represent 80% of relative frequency (Grammatophora marina, Cocconeis speciosa, Navicula longa, Amphora sp., Cocconeis dirupta, Tabularia investiens, Cocconeis plancentula, Cocconeis distans, Tabularia fasciculata, and Cocconeis notata). These findings suggest that Fissurella volcano is a herbivore with preference for diatoms, periphyton, and foliose and filamentous red seaweeds.