Two Nostoc species were used to study the toxic effects of two commonly used insecticides: sumi-alpha [(s)-α-cyano-3-phenoxybenzyl (s)-2- (4-chlorophenyl)-3-methylbutyrate] and Dursban [O,O-diethyl-O-(3,5,6-trichloro-2-Pyridyl) phosphorothioate) ] to soil microflora. The heterocystic filamentous Nostoc minutum Desm. ex. Born. et Flah.& Nostoc puniforme Ag. ex. Born. et Flah. were exposed to different concentrations of sumi-alpha and dursban. Differential growth effects were observed on the two cyanobacterial species exposed to different insecticide concentrations. Selected parameters of the tested cyanobacteria (dry weight, Chl a, photosynthetic O2 evolution, total soluble sugars, nitrogenase activity, total soluble proteins, total lipids and fatty acid contents of the total lipids) were measured and examined. The data revealed a marked inhibitory effects in all measured parameters in both Nostoc species under different insecticides concentrations of sumi-alpha and dursban except the total lipids. This suggests a potential hazard of these insecticides on soil microflora and subsequently natural ecosystem.