The aim of this study was to investigate seasonal variability in the growth and size structure of dominant phytoplankton species in Buyukcekmece Reservoir, and to determine the environmental factors that affect morphological plasticity in those dominant species. In total, 78 taxa from 8 algal divisions were identified. Bacillariophyta and Chlorophyta were the most important divisions with respect to Species number, but cryptomonads Plagioselmis nannoplanctica (Skuja) Novarino, Lucas, and Morrall, and a Cryptomonas sp. were dominant in terms of density and biomass. P nannoplanctica was the major contributor of biomass in the reservoir. A significant difference in Plagioselmis species biovolume was observed between winter-early spring and summer. A significant negative correlation was observed between its seasonal size structure and abundance. During winter (December 2004-April 2005) nutrient values were higher than during autumn (October-Novermber 2005) and P. nannoplanctica had maximum cell size (17.5 mu m x 10 mu m; biovolume: 589.5 mu m(3)), but its abundance was very low (< 6 cells x 10(-3) l(-1)). The minimum surface:volume ratio (SA/V:87) was recorded in this period. Among the measured environmental factors, water temperature was the only parameter that had a statistically significant negative relationship (R-2:0.635; P < 0.01) with P. nannoplanctica cell size. The results indicate that seasonal changes in cryptomonad size structure were controlled by water temperature.