The gonadal organization and cytological characteristics of gametogenesis in a population of the freshwater clam Musculium argentinum (d'Orbigny, 1835) are described. Sections of the rather small gonad demonstrate the existence of hermaphroditic specimens, and is composed of hermaphrodite follicles among the intestine coils in the posterior dorsal region of the body. Follicles contain male and female germ cells at different stages of maturation, which can be recognized by their shape, size, and nuclear features. Male germ cells are organized in cysts, and spermatocytes do carry out meiosis. Within follicles, male and female germ cells occupy different regions. Gonad activity throughout the year is coincident with the life cycle pattern of the M. argentinum population studied previously.