A population of the viviparous freshwater bivalve Sphaerium corneum L. from an artificial mill race subjected to irregular changes of water level was investigated using quantitative monthly sampling, paraffin histological sections and chromosome preparations. Unlike most of the previously studied populations of the genera Sphaerium and Musculium, that presented in this paper exhibits less synchronised life-spans. Despite two main birth periods, many specimens also release their broods individually during the season. An adult usually breeds at least twice in a season. The life span is more than one, usually one and a half years. All age and length classes are able to overwinter. Spermatogenesis and oogenesis apparently proceeds simultaneously from spring to autumn and they occur also in fully developed larvae which are still retained inside their parents' gills. Relationships between the observed breeding strategy and fluctuations of the environment are discussed, as well as the intraspecific and interspecific variation in life-history traits within the Sphaeriidae.