The species composition, abundance, and biomass of micro- (>15 μm) and nano- (<15 μm) phytoplankton were studied along the southern Black Sea during June-July 1996 and March-April and September 1998. A total of 150 species were identified, ̃50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77×10 3 cells l -1 in spring to 110×10 3 cells l -1 in autumn and biomass from 250 μg l -1 in summer to 1370 μg l -1 in spring. Based on the extensive sampling grid from June-July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin.