Freshwater mussel Dreissena polymorpha is one of the most dominant invasive species in the rivers and lakes of Europe, which began to spread in Croatia in the 1980s. To complement the knowledge about reproductive characteristics of this species in the accumulation lake of Dubrava Hydroelectric Power Plant, we analysed females sampled in the reservoir from March to August 2000. The objectives of the study were to investigate the seasonal patterns of oocyte growth and growth of their nuclei, compare the gonad developmental stages with recorded sizes of oocytes and to associate the obtained data with the results from previous research in Croatia. The development stage of gonads and the growth of oocytes were analysed on visceral mass tissue, processed for paraffin blocks technique, cut to a thickness of 10 μm and stained with hemalaun-eosin. The results show that females sampled in March are at the stage of gametogenesis, spawning lasts from April to June, in July females are in the post spawning stage, and in August animals are in resting stage, with the signs of new gametogenesis. In total, we measured 5190 oocytes and their nuclei, by using morphometric program Zen2Lite. The highest average value of the oocytes diameter and of their nuclei (35.32 μm; 23.02 μm) was recorded in June (the gonadal stage before spawning), while the lowest average value (10.34 μm; 6.62 μm) was noted for the resting stage in August. Statistically significant differences in the diameter of oocytes and their nuclei were recorded among the studied months and also among established gonadal stages (p < 0.001). Identified reproductive characteristics of this species are not significantly different from previous studies in the Drava River and reservoir Dubrava, while minor differences in relation to the accumulation of Čakovec can be explained by environmental factors, primarily water temperature.