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The synthetic pyrethroid insecticides cis-and trans-permethrin are widely used, particularly in sheep-dips and for mothproo®ng within the textile industry. This study aims to establish the concentrations and mobility of permethrin within rivers in the Humber catchment, which contain some of the highest densities of wool-scouring activity in the world. Our approach was to utilise three different surveys: (a) weekly and storm-responsive sampling of`whole waters' in the rivers Aire, Ouse, Don, Trent and Calder; (b) intensive sampling of`whole waters' and suspended sediments in the rivers Aire (0.5 h) and Calder (1 h); (c) a bed-sediment survey of the River Calder. Sediments were extracted by supercritical¯uid extraction and all samples were analysed by gas chromatography with mass spectrometry.Results demonstrated the presence of permethrin in`whole waters' and sediments, particularly in the rivers Aire and Calder, and at concentrations in the bed-sediment likely to cause ecotoxicological effects to benthic macro-invertebrates. Mass-balance calculations indicated some loss of permethrin from the water column during transport, with the greatest losses at low river-discharge. Isomer ratios (trans:cis) give retention times of permethrin in different components of the system as`whole water'`suspended sediments`bed-sediments, with estimates of 4±26 days for suspended sediments and a maximum of 103±125 days for surface bed-sediments.