Eucaryotic picophytoplankton (PEUC), picocyanobacteria, and larger phytoplanktonic cells from the Thau Lagoon (northwest Mediterranean coast) were numbered by flow cytometry from November 1991 to February 1994. PEUC cells dominated the phytoplanktonic assemblage and exhibited seasonal dynamics. Monthly mean abundances of larger phytoplanktonic cell and PEUC were significantly correlated with temperature and solar irradiance values, with the highest correlations having been obtained for PEUC abundances. The mean abundance of total picophytoplankton (3.5 × 104 cells ml‒1) was among the highest recorded in marine waters. The dominance of picophytoplankton over larger phytoplanktonic cells (average abundance, 5 × 103 cells ml‒1) in the nutrient‐rich waters of the lagoon was related to grazing from large‐scale shellfish breeding (oyster’s ≈35,000 t), which seems to act preferentially on the largest cells. Several hypotheses, including the potential selective effect of copper, were proposed to explain the dominance of the PEUC form (average abundance, 3.4 × 104 cells ml‒1).