The wavy turban snail Megastraea undosa and keyhole limpet Megathura crenulata, two gastropods with commercial importance, five macroalgae, and one seagrass were collected at Bahía Tortugas, Baja California Sur, Mexico, for analysis of stable isotope ratios. The δ¹³C and δ¹⁵N isotope signatures obtained from gastropod muscle tissue were different between the two gastropods. Isotope signatures of M. crenulata had more enriched δ¹³C (-14.35‰ ± 1.66‰) and δ¹⁵N (15.48‰ ± 0.67‰) than M. undosa isotope signatures for δ¹³C (-17.59‰ ± 2.43‰) and δ¹⁵N (12.42‰ ± 0.61‰). Seaweed isotope signals varied from -21.8‰ to -11.4‰ for δ¹³C and 9.3‰ to 10.9‰ for δ¹⁵N. The δ¹³C of detritus was -7.93 and of tunicates was -9.3‰ and the δ¹⁵N of detritus was 12.4 and of tunicates was 8.1‰. Mixing models showed that the greatest contribution to the diet of M. undosa was from Prionitis cornea and Gelidium robustum. For M. crenulata, it was from detritus and Macrocystis pyrifera. A higher trophic position (3.1) was observed for M. crenulata than M. undosa (2.3), which clearly reflected the differences in their feeding habits: M. undosa is a herbivore and M. crenulata is an omnivore.