Effect of Monoformamidine insecticide,N'-(2,4-dimethylphenyl)-N-methylformamidinehydrochloride(DMAH),on the aquatic community structure were experimentallyinvestigated through glass fibre glass fibre tanks with a dimension of 3m×1m×1m(V=3m)under outdoor conditions in Beijing,1993. The nominal concentration of 0,1.5, 3. 0, 6.0 and 12.0mg· L-1 were dosed to test tanks,respectively, once two weeks. The experiment ran for 2months. The results indicate that the DMAH was toxic to aquatic community. Plankton comun...