Phytoplankton temporal fluctuation and vertical distribution were studied by seasonal and close interval siphon sampling (May-Sep) in a shallow lake dominated by common water milfoil (Myriophyllum sibiricum). Factors potentially regulating phytoplankton primary production were investigated in situ in 2 enrichment bioassays. The results suggest that the macrophyte vegetation provided an unfavourable habitat for large colonial chlorophytes. Small species, mainly cryptophytes but also small chlorophytes and cyanobacteria, characterised the summer phytoplankton. Phytoplankton abundance as well as primary production were considerable and remained in the mesotrophic range. The total nitrogen to total phosphorus ratio during the growth season, and the enrichment bioassays, showed that phosphorus was a significant regulator of primary production. Small-celled species had competitive advantages in the dense vegetation, and the canopy structure created by dense stands of common water milfoil allowed phytoplankton growth. Hence, although the macrophytes altered the physical and chemical conditions in the lake, coexistence of small-celled algae and macrophytes was possible.