In Australia the primary pesticide for control of heliothis in cotton is the organochlorine endosulfan. In recent years it bus been supplemented by a variety of other pesticides including organophosphates, carbarnates, pyrethroids and trisect growth regulators in an effort to reduce the level of pesticide resistance. Endosulfan is toxic to aquatic life (either as the two parent isomers or as the breakdown product, endosulfan sulfate), with fish being reported as highly sensitive to the chemical. Concern over the environmental fate of cotton pesticides in the late 1980s, led the New South Wales Department of Land and Water Conservation DLWC) to undertake in 1992 biological monitoring of aquatic macro invertebrates to assess the impact of irrigated agriculture on the rivers in the north west region of New South Wales (Figure I). The work forms part of the Central and North West Regions Water Quality Program (CT. IWRWQP) which is jointly funded by the DLWC and the water users of the Gwydir, Macquarie, Namoi and Border Rivers valleys. The other major components of the CN\NRWQP focus on pesticide, physico-chemical and nutrient monitoring.