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Abstract:The gut content was analyzed in order to clarify the potential food sources of the oyster (Crassostrea gigas) and the clam (Ruditapes philippinarum) in the Akkeshi-ko estuary. Benthic diatoms in the diets of oysters were most abundant and accounted for 70% in 2003 and 67% in 2004, followed by pelagic diatoms and dinoflagellates. In gut of clams, benthic diatoms represented 78% in 2003 and 87% in 2004, also followed by pelagic diatoms and dinoflagellates. There are no differences in dietary preference between oyster and clam. There are no significant correlations between phytoplankton compositions in gut of oyster/clam and water column. The Index of electivity was high particularly in dinoflagellates and several species of benthic diatoms such as Amphora sp., Cocconeis scutellum, Nitzschia sp., Paralia sulcata, Rhoicophenia curvata and Thallasiosira sp., Gramatophora marina, and so on.