Species composition, abundance, and biomass of micro- (>15 mu m) and nano(<15 mu m) phytoplankton were studied in the western and eastern Black Sea during March-April and October 1995. A total of 142 species were identified, of which >50% were dinoflagellates. Abundance and biomass values were lower during the March-April period (average 129 +/- 28 thousand cells I(-1) and 330 +/- 124 mu g I(-1)) than during the October period (average 364 +/- 161 thousand cells I(-1) and 1794 +/- 515 mu g I(-1)) and compared with previous investigations. Values for the north-westerly region were higher than for the southerly areas, probably owing to effects of the Danube river, but were much lower than previously reported, possibly indicating improved ecological conditions. In March-April, dinoflagellates (mainly Heterocapsa triquetra and Scrippsiella trochoideum) were the most important groups, whereas, in October, diatoms (mainly Pseudosolenia calcar-avis) and coccolithophores (Emiliania huxleyi) were dominant. Nanophytoplankton constituted 57% and 84% of total abundance and 8% and 3% of total biomass in spring and autumn, respectively. Microphytoplankton were dominant in the western Black Sea, whilst nanophytoplankton were dominant in the eastern region in spring. (C) 1999 International Council for the Exploration of the Sea.