Littoraria ardouiniana and Littoraria melanostoma are common snails in Hong Kong, living and feeding on mangrove trees. Gut content and stable isotopic analyses were conducted to investigate the littorinid's diets. Gut content analyses revealed these snails ingested bark, epidermal plant cells, fungi, and microalgae, but that broken plant cells were the most abundant food items in the stomach and faecal contents. The gut contents of the two littorinid species, either from the mangrove trees Kandelia candel or Aegiceras corniculatum, were similar and showed little temporal variation throughout the year. Dual stable isotopic analysis, which investigated the δ13C and δ15N values of the littorinids and their potential food items, indicated that these littorinids might feed on mixed diets composed of parts of the mangrove trees and other items available on the trees such as phylloplane fungi, microalgae and cyanobacteria. These epiphytic mangrove littorinids are generalist grazers which graze on the substratum non-selectively as they are constrained on their host trees, and their diets are, therefore, dependent on food availability on the trees themselves.