We determined the stable carbon and nitrogen isotope compositions of foot tissue, conchiolin opercula, and the shell organic matrix from the marine gastropods Cernina fluctuata (Ampullinidae) and Polinices mammilla (Naticidae) collected in March 2006 from a lower tidal flat on Cuyo Island, the Philippines, together with the seagrass Enhalus acoroides and the macroalga Padina australis. The data from the foot tissue samples confirmed that the food sources for C. fluctuata were autotrophs, such as macrobenthic marine plants and algae, whereas P. mammilla may derive carbon from epiphytes and phytoplankton via their bivalve prey. The δ¹³C and δ¹⁵N values for C. fluctuata foot tissues from specimens collected in 1995 were similar to those from specimens collected in 2006. The δ¹³C values for C. fluctuata shell organics collected in 2006 were approximately 3.6‰ higher than those for P. mammilla shells, which was a similar trend to the relationship for δ¹³C values observed in their foot tissues. However, the mean δ¹³C values for the shell organics from the specimens collected in 1995 showed no significant differences between C. fluctuata and P. mammilla. In all the gastropod specimens analyzed, we observed a large (at least 10‰) δ¹³C depletion in shell organics with respect to the foot tissues of C. fluctuata and P. mammilla, although the reason for this is unclear. The δ¹³C values for the P. mammilla conchiolin opercula were similar to those for the foot tissues, whereas there was a larger δ¹³C depletion in conchiolin opercula with respect to the foot tissues of C. fluctuata. Therefore, gastropod conchiolin opercula should not be used for short-term dietary analyses in some gastropod species, but they may rather reflect the integrated long-term dietary trend.