The chlorococcalean flora from two crossed rivers during the summer period (from July to September 2001 and 2002) was examined. Phytoseston communities were collected from the We?na and Nielba rivers before and behind the cross-rivers as well as from the central place of the cross-rivers. Species belonging to 6 dominant genera: Scenedesmus, Pediastrum, Monoraphidium, Ankistrodesmus, Coelastrum and Tetraedron were found in both rivers. At most of the stations located along the We?na and Nielba rivers after their crossing a trend of increasing green algae species richness together with an increase in the total species number of cells was observed. Furthermore, it was found that the We?na river takes over some of the Nielba phytoseston species.