With the aid of epifluorescence microscopy, picoplankton (0.2-2mum) in the coastal zone of the Gulf of Gdansk (southern Baltic) was studied from March to December 1997. The main portion of the picoplankton biovolume was comprised of bacteria, making up on average 70% of picoplankton. Autotrophic picoplankton (APP) prevailed over the bacterioplankton only in late August and early September. The main components of APP were cyanobacteria. The APP proportion in the total phytoplankton biomass was highest in summer. The assessment of this share in terms of quantity varied depending on the picoplankton separation method. In the microscopic analysis of unfiltered samples, all cells <2mum were counted as picoplankton, including colony-forming types. Based on the analysis of unfiltered samples, the amount of APP in the overall phytoplankton biomass was assessed at 16-22% in summer, and 0.5-6% in the remaining period. Initial filtration of samples through a 2mum filter caused great losses in APP, resulting mainly in the elimination of colony-forming cyanobacteria cells from samples. Prefiltered samples were also contaminated by cells >2mum. In prefiltered samples the APP proportion in the total phytoplankton biomass was 1-7% in summer and 0.2-3% in the remaining period.