Crayfish have “gastroliths” in which calcium is deposited transiently during the molt cycle from the cuticle. In red swampcrayfish, amorphous calcium carbonate is embedded in an organic matrix composed mainly of chitin. Thus the seasonalityof molting will strongly affect the number and possession rate of gastroliths in crayfish. In this study, we captured 405 wildred swamp crayfish in May-November 2014 at a small pond and stream in Tsuzuki central park, Yokohama using a net forcapturing crab or a fishing line with dried squid, and analyzed the seasonality of number of gastroliths in their stomach,degree of calcification of the gastrolith in relation with the molting cycle. Among 405 crayfish, only 48 individuals had one ortwo gastroliths, in a biforked stomach. Gastroliths were found only in May-July and October-November, and large gastrolithswere found from the individuals just before and after the molt. We also found one or two chitin matrix from the stomach ofall the individuals who didn’t have gastroliths. Red swamp crayfish are important food item of fish-eating birds such as greatcormorant, and their gastroliths and chitin matrix are found in the birds’ pellets. Here we propose a formula to estimate theintake of crayfish by birds using the number of gastroliths and chitin matrix found in birds’ pellets.