Abstract The invasive bivalve species Dreissena polymorpha (Pallas, 1771) has become one of the most dominant spe-cies in many lakes and rivers of Europe and North Ame-rica. The aim of this study was to investigate changes in the structure of the visceral mass depending on the repro-ductive cycle and to determine the presence and effects of trematodes on the zebra mussel. Mussels were collected from the Dubrava dam reservoir during eight sampling sessions from January to December 2000. For histological analysis, 80 mussels were placed in Bouin’s fixative for 24 hours, embedded in paraffin and cut with a microtome into 10 μm slices. The sections were then stained with haema-toxylin and eosin. Analysis of non-infected bivalves indi-cated that gametogenesis started in late summer, while spawning began in April. In July, the majority of mussels were in the post-spawning stage. Two mussels were in-fected with Bucephalus polymorphus, one with Echi-noparyphium recurvatum and one with Aspidogaster lima-coides. Obtained results demonstrated that the gonads were the most affected organs in mussels infected by B. poly-morphus, the infection by A. limacoides resulted in a sig-nificant reduction of all investigated organs, while the infection by E. recurvatum had no visible pathogenic changes in the structure of visceral mass.