In this study, we examined microplastic contamination in the gills and guts of two filter-feeding oyster species, Crassostrea gigas, and Crassostrea nippona, in Tokyo Bay, an area of high-level MP pollution, to determine the extent of contamination. The mean (± SD) concentration of MPs in C. gigas in spring was 29.1 ± 19.9 MPs g-1 WW in the gills and 181.0 ± 229.7 MPs g-1 WW in the guts. For C. nippona, MP concentration in the gills and guts during spring was 144.5 ± 77.1 MPs g-1 WW and 149.7 ± 123.4 MPs g-1 WW respectively. In summer, C. nippona had a mean (± SD) MP concentration of 193.3 ± 66.9 MPs g-1 WW in the gills and 302.8 ± 232.1 MPs g-1 WW in the guts. MP uptake was significantly higher (p < 0.05) in C. nippona than in C. gigas in spring. Fragment was the dominant shape with a size frequency of 40-60µm. PE was the most abundant polymer except in the gills of C. gigas, where PA was dominant. MP Polymer composition in oysters corresponded to those in the seawater.