The use of methoxychlor in the Athabasca River to control black fly (Diptera: Simuliidae) larvae was monitored in 1980 by Alberta Environment. Effective control of black fly was observed for a distance of 60 km. The effect of the methoxychlor on non-target invertebrate organisms was considerable at sites located 60 km and closer to the treatment points, however, complete recovery was observed over the summer. Methoxychlor residues in the silt bedload dissipated fairly rapidly after treatment. Adult black fly activity data indicated that adults were more abundant in August than in June, suggesting that methoxychlor treatment of the river in late May had an effect on reducing adult numbers in the farming area during June.