Hermaphroditic Corbicula leana clams reproduce by androgenesis and have been regarded as simultaneous hermaphrodites. To date, there has been no report on the occurrence of male clams in hermaphroditic Corbicula. In an irrigation ditch in Shiga Prefecture, we found that 78.2% of C. leana specimens were males and 21.8% were hermaphrodites. Microfluorometric analysis revealed that males were diploids and hermaphrodites were triploids. All males produced nonreductional and biflagellate spermatozoa. The sequence analysis of mitochondrial DNA (cytochrome b, 621 bp) for 31 specimens of C. leana showed that four male and nine hermaphrodites shared the same H2 mtDNA haplotype; H1 was detected from 17 males and H3 was detected from one hermaphrodite. Coexisting C. fluminea clams also have haplotypes H1 and H2. Phylogenetic tree by a neighborjoining method based on the partial sequence of cytochrome b revealed that the haplotypes (H1– 3) of C. leana were evidently different from those of dioecious C. sandai (S1 and S2) and C. japonica (J1 and J2). These results suggest that males may be derived from hermaphrodite C. leana clams. The role of males in hermaphroditic populations is unknown. However, if the spermatozoon from a male is able to fertilize an egg from a hermaphrodite and the nuclear genome of the egg is expelled as polar bodies, the sperm nucleus could form a zygote nucleus. This mode of reproduction would allow the replacement of the nuclear genome.