A dense bloom of picocyanobacteria with biomass of 10-50 g m-3(wet weight) and numerical density 5-20 × 106 cells ml-1 broke out in the Lagoon of Venice in July 2001. The share of picocyanobacteria of the total phytoplankton varied in the Venice lagoon in July-September from 60 to 98% depending on the vicinity of the site to the channel's driving tidal currents. The washout of the picocyanobacterial biomass occurred during the ebbs to the shelf zone of the adjacent Adriatic sea. The biomass of picocyanobacteria in coastal Adriatic water was at that time up to 1.7 g m-3(w.w.) with the share of picocyanobacteria ranging from 70 to 90%. The rest consisted of small phytoflagellats. The contents of suspended and labile organic matter in water increased during the bloom by a factor of 5-15. The photosynthesis rate in upper water layers rose by about 2 orders of magnitude, attaining 3-5 g C m-3 day-1, with a decomposition rate of 2-3 mg O2 l-1 day-1. The residence time of inorganic phosphorus standing stock in water was found to be as short as 6-12 min. The populations of micro- and mesozooplankton were found to be inhibited in areas of intensive bloom. A significant mortality of key species for the local fishery, e.g. the Manila clam, Tapes philippinarum, was recorded in the lagoon in September-October.