Drift of aquatic insects was compared at three sites downstream (21, 38, and 107 km) from methoxychlor treatment (0.3 mg. L−1 for 15 min) of the North Saskatchewan River, relative to an upstream untreated site. Species of Diptera (Simuliidae), Ephemeroptera (Baetidae, Heptageniidae, Ephemerellidae, Ametropodidae, Metretopodidae, and Tricorythidae), Plecoptera (Perlodidae and Chloroperlidae), Trichoptera (Hydropsychidae and Hydroptilidae), and Hemiptera (Corixidae) were studied. Drift responses differed depending on species, distance from the injection site, and time after methoxychlor injection. Exposure to methoxychlor initiated catastrophic drift of aquatic insects at all downstream sites. Of 22 species compared before treatment and following methoxychlor injection, post-treatment drift of 17, 21, and 13 species significantly exceeded pre-treatment drift at the km 21, 38, and 107 sites, respectively. Methoxychlor treatment initiated or increased drift of several normally non-drifting species. Similar drift patterns were observed among closely related taxa during the catastrophic phase. For all species studied, comparisons of 24-h drift densities between days preceding and following the catastrophic phase of treatment indicated significant post-treatment drift density increases or decreases at one or more of the downstream sites, but not at the untreated site. Species were classified according to their drift responses to methoxychlor treatment. Factors that may have caused different drift responses among species are discussed.