Numerous (0.5 to 4.8 × 10 5 cells/ml), small phytoplankton (smaller than 0.5-1 × 1-2 µm in cell size, picophytoplankton) were distributed in the halocline (depth 2-12 m, 4-14 practical salinity units) of the saline meromictic lake, Lake Suigetsu (35°35′ N, 135°52′ E), located in the central part of the coast of Wakasa Bay along the Japan Sea in Fukui Prefecture, Japan. Vertical distribution of phytoplankton revealed that the maximum number of picophytoplankton was always observed near or a little deeper than the oxic-anoxic boundary layer (depth 5-6 m); they were dominant phytoplankton in the water layer deeper than the oxic-anoxic boundary from July to late September 2005. Spectral analysis of autofl uorescence emitted from the particle fractions smaller than 5 µm measured with a spectrofl uorometer and from individual cells measured with a microscope photodiode array detector revealed that the major component of picophytoplankton was phycoerythrinrich, unicellular cyanobacteria (picocyanobacteria). Eukaryotic phytoplankton about 2.5 µm in diameter were also found, but the numbers were low. Fluorescence intensity of chlorophyll a at 685 nm (room temperature) emitted from the particle fractions smaller than 5 µm was increased by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These observations indicated that at least some picophytoplankton had a functional photosystem II in the halocline where sulfi de, the potential inhibitor of oxygenic photosynthesis, was always present. The large abundance together with their physiological potency suggest that picophytoplankton are one of the important primary producers in the halocline of Lake Suigetsu.Electronic supplementary material Supplementary material is available in the online version of this article at http://dx.