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.