Gut content analyses and stable isotopes (δ13C and δ15N) were used to investigate the food consumption and assimilation of the pulmonate, Onchidella nigricans, within a rocky intertidal platform at Waiwera, northern New Zealand. Analyses of gut contents indicate that this species is a generalist herbivore, which may consume a variety of micro- and macro-algae, although small zooplankton may be ingested, when present. Gut contents of individuals collected from different intertidal habitats (bare rock, Hormosira banksii, green filaments, and coralline algae) reflected the dominant algal species within each habitat, suggesting that food availability does not restrict the grazer's distribution across its range. The radular morphology and small size of this gastropod also support the notion that O. nigricans is a non-selective microphagous feeder. However, stable isotopes on O. nigricans from the four habitats and the dominant algal food types indicate a strong assimilation preference for microalgae. The δ13C values (-15 to -13‰) of O. nigricans were consistent with signatures for intertidal grazers, but δ15N values (8 to 9‰) were relatively high, which may indicate the presence of bacteria and microfaunal detritus in the diet. Clusters of isotopic signatures of individuals from different habitats suggest feeding preferences, which also may be attributed to differences in detrital and bacterial consumption. This study illustrates the importance of using parallel techniques in diet studies.