The Pesticide Chemicals Branch of Alberta Environment conducted a monitoring program in 1979 related to two methoxychlor treatments of the Athabasca River for black fly (Simulium arcticum) control. Three populations of black fly larvae were observed in 1979 and the first two populations were reduced by 91.2% (June 7 treatment) and 98.1% (July 11 treatment) respectively. Population reductions of nontarget invertebrate organisms was observed but recovery was fairly complete within 4 weeks of treatments. Silt samples collected over the summer indicated that methoxychlor did not accumulate in the river bottom silt. Water samples that were collected for the July 11 treatment at Fort McMurray (250 km downstream of treatment point) indicated that maximum concentrations of methoxychlor present in the river water was 3.5 ppb. Adult activity sampling indicated that there was a fairly close relationship between adult emergence expected through larval development data and adult activity measured along the river.