Sex, maturity, and the incidence of males in a population of hermaphroditic Corbicula fluminea Müller from Shirakawa River, Kyoto, Japan, are reported. Two shell color types (yellow and green) of this species are recognized. High frequencies of females (63.3% yellow and 58.5% green) are represented among individuals in this population. Histological examination of gonads and measurements of shell length suggest this species first matures as a female before commencing spermatogenesis (i.e., it is a protogynous simultaneous hermaphrodite). Low frequencies of males (mostly less than 5%), the testes of which are filled with spermatozoa, are reported; no histological evidence for any change from males to hermaphrodites, or hermaphrodites to males, is found. Requirements for maintaining males, and for their successful reproduction in self-compatible hermaphrodite populations, are proposed as follows: (1) the protogynous nature of the hermaphrodite, (2) low self-fertilization rates of hermaphrodites, (3) more spermatozoa and greater survival of males than hermaphrodites, and (4) high population densities, ensuring males and hermaphrodites are in close proximity.