A single 7.5-min injection of 0.6 p.p.m. of methoxychlor into the North Saskatchewan River eliminated 100% of instars 3 to 6 inclusive of the larvae of Simulium arcticum Mall, from sites 40 and 80 km downstream, 91% at 121 km, and 66% at 161 km. Ninety-six per cent of the larvae less than 1 mm long were removed from the 161-km site. Plecoptera larvae were similarly affected. Chironomidae, Ephemeroptera, and Trichoptera were least affected.

The treated portion of the river was rapidly repopulated by insect larvae. None of the pretreatment taxa disappeared and in fact many new ones appeared, during the 10 weeks immediately following the treatment. In all four sites examined in the treated portion of the river, population densities of Chironomidae larvae larger than 1 mm long equalled or surpassed the pretreatment densities within 1 to 3 weeks, Ephemeroptera within 1 to 4 weeks, Trichoptera 1 to 7 weeks, Plecoptera 4 to 5 weeks, and Simuliidae in 2 to 10 weeks. Populations of larvae smaller than 1 mm long were generally restored more rapidly. Procedures for an annual abatement programme for S. arcticum in the Saskatchewan River are outlined.