Yamato-shijimiclam Corbicula japonica is the best-known bivalve inhabiting ln brackish estuaries and lakes around Japan and one of the most commercially important species in inland fisheries. Althoughthe amotmt of C japonica from Lake Shinji has accotmted for the large part of the domestic catch, itsamolmt has dramatically fallen in recent years・ This study was conducted to verifythe genetic structtue and reproduction mechanism of C・ japonica, both of which are essential to its stock management in Lake Shinji. Weanalyzed sequence polymorphism of the 588 bp portion ofthemitochondrial DNA cytochrome oxidase subunit I (Col) gene to determinethe population genetic structure of C・ japonica in Lake Shinji・Among a total of 177 C・ japonica specimens collected from 4 colonies, 37 haplotypes were obtained, and 2 major haplotypes were apparent withrelatively highabtmdance in all colonies・ Well correspondingmismatch distributions along the Col gene were determined for the data sets of the individual colonies, and the pairwise population estimates FsT among the individual colonies were also generally low, Such small genetic differentiation of C japonica is derived kom highgeneflow in Lake Shinji, and this could be caused by a lake-wide dispersion of its larvae mediated by the water movements・Article信州大学山地水環境教育研究センター研究報告 6: 115-124(2010)departmental bulletin pape