As a common sessile filter feeder with high bioconcentration rates, Dreissena polymorpha is useful as a bioindicator species. Pollution and environmental factors influence the reproductive effort of these bivalves. Therefore, the evaluation of the gametogenic cycle is important for understanding environmentally-related physiological alterations in this dioecious species. Animals from an unpolluted reference site close to the highly polluted Seine River, France, were monitored monthly over 1 year using qualitative and quantitative measures of gametogenesis. Developmental stages of the gonad were determined by visual inspection of histological slides. Unbiased quantitative stereological analysis was conducted to estimate oocyte development and the gonad index was determined. To investigate a possible role of female sex steroids in bivalve gametogenesis as a possible target for xenoestrogens, free oestradiol levels were assessed and analysed with respect to oocyte development. The basic pattern of the reproductive cycle consisted of early gametogenesis during autumn and winter, followed by rapid vitellogenesis in early spring, and spawning towards the summer months. However, the population was characterized by high inter-individual variability of the gonad stages. Furthermore, the simultaneous occurrence of different developmental stages within the same animal made classification difficult. Stereological determination of oocyte volumes as well as gonadosomatic index provided a more precise evaluation of reproductive effort. Oestradiol levels did vary significantly between months, but did not show any clear relationship with a particular stage of gametogenesis or oocyte volume.