Desmodesmus spinosus isolated from Lake Balaton (Hungary) and Scenedesmus obliquus isolated from the Nile water (Egypt) were studied in continuous cultures at 25 °C and light intensity of 210 µmol m^-2 s^-l under different growth rates. At the steady states abundance, dry weight, chlorophyll-a, morphology and dimensions of cells were measured. The maximum growth rate was 2.78 ±10.1 and 0.93 ±0.01 d^-l  and the half saturation constant of growth was 7.18 ±11.0 µP l^-1 and 53.76 ±17.4 µg N l^-1  for  D. spinosus and S obliquus, respectively. The cell volume and cell morphology varied with the growth rate. Cell size and chlorophyll content per cell or per cell volume increased with growth rate. Both species were isolated as a 4-celled coenobium and at time of isolation identified as D. spinosus and S obliquus. The 4-celled coenobia completely disappeared and only unicells algae were observed at low dilution rates. Rising of growth rate formed two-celled and/or 4-celled coenobia. At growth rates near or close to the maximum growth rate only four-celled coenobia (spiny in case of D. spinosus) were found in the culture vessels and they had form and dimensions the same as that found at the time of isolation. At low growth rates the morphological characters of formed unicells may overlap where the limiting nutrient concentration was very low. The plasticity of both species may depend on the growth and environmental conditions and it may cause misidentification for these species in natural samples.