Sphaerids have adopted a variety of life strategies that have allowed them to show a cosmopolitan distribution. Among these strategies, those related to reproduction, such as sexuality (hermaphroditism), offspring incubation, fertility, number of generations per year, times an individual reproduces during its lifespan, among others, have been decisive with regard to its permanence in time and space, and to colonize and maintain populations in permanent or temporary freshwater bodies. The aim of the current study is to compare reproductive traits of 2 Musculium argentinum populations inhabiting different environments (stream and lake) to determine the potential influence of the environment on reproduction of these populations. During 1 y (October 2006 to 2007) monthly samples were taken at 2 study sites: Lautaro stream (38°32′ S, 72°27′ W) and La Poza, Villarrica Lake (39°16′27″ S, 71°58′54″ W). Sixty specimens were collected monthly from each population. Valve length of each specimen of the sample was registered to determine the size structure of each population. Fifty animals of known valve length were dissected to determine presence and number of brooding sacs in the inner hemibranches, number of embryos in each sac, and number of newly hatched clams. Ten specimens were processed for optical microscopy with routine histological techniques to characterize the gonad activity pattern. Results indicate that M. argentinum is hermaphroditic in both populations, with gonadal activity throughout the year, iteroparous, and with embryos in brooding sacs and newly hatched clams present year-round. These results indicate that differences in the environment did not produce major changes in reproductive strategies in both M. argentinum populations studied.