Food sources for the deposit-feeding gastropods Assiminea japonica and Angustassiminea castanea (Gastropoda: Assimineidae) were estimated using stable carbon and nitrogen isotope ratios. We collected animals and potential food materials in reed marshes of the Nanakita River estuary, the eastern part of Miyagi Prefecture, Japan. Feeding experiments were also conducted to confirm whether snails assimilate 4 types of diets. The stable carbon isotope ratios of both assimineid species (mean +/- 1 SD: A. japonica, -20.7 +/- 0.3parts per thousand; A. castanea, -19.8 +/- 0.5parts per thousand) were closer to that of deposited organic matter from lagoon water (-20.7 +/- 0.3parts per thousand) than to those of reed litter (-25.4 +/- 0.1parts per thousand) and the surface soil of the reed marsh (-26.3 +/- 0.1parts per thousand). The snails that were fed deposited organic matter showed delta(13)C values similar to the control animals before feeding experiments for both species. The delta(13)C values of the snails fed litter or soil diet, however, revealed that these snails were able to assimilate organic matter from reed detritus under laboratory conditions. These findings suggest that the salt marsh snails utilized mainly deposited organic matter from lagoon water in the field. Microalgae such as phytoplankton and benthic diatoms in deposited matter are considered to be important food sources for 2 species of assimineids inhabiting salt marshes of the Nanakita River estuary.