In this paper, polyploidy is reported to occur in three species of Korean Sphaeriidae, Musculium cf. lacustre, Sphaerium cf. corneum and Sphaerium sp. The mitotic chromosome numbers of M. cf. lacustre and S. cf. corneum are both 190, and Sphaerium sp. has a mitotic chromosome number of 171. The basic chromosome number of these Sphaeriidae is probably x=19. Because of the large number of chromosomes present it is difficult to determine ploidy, but M. cf. lacustre and S. cf. corneum may be decaploid and Sphaerium sp. may be nonaploid.