The organic excretion of non-sterile monoalgal cultures of Scenedesmus in- crassatulus grown in fresh and Black Sea water were investigated using gas chromatogra- phy/mass spectrometry. Lactic acid predominated, oxalic, hydroxybutiric, succinic acids, and some organic phosphates were also found. Dehydroabietic, 2-hydroxy-3-(4-hydrox- yphenyl)-propanoic, N,N-Dimethylglycine, N,N-dimethylisopropenylglycine were iden- tified, they are new found substances in algae. Algal cultures at exponential growth phase, which was characterized with high density around 4 g.dm-3, excrete relatively less organic matter than the phytoplankton in natural ponds or starving algal cultures in model experiments.