Preliminary tests have been conducted in northern and southern Quebec on the use of temephos for the control of the larvae of Simulium venustum Say, the main man biter in those regions. The larvae are usually concentrated in the first 30 m of streams below lake outlets and control has to be done when the water temperature is around 8C or 10C. Our tests at 0.3, 1 and 3 ppm for 20 min with the emulsifiable concentrate of temephos in ground application show that efficiency of the larvicide immediately below site of application could be closely related to water temperature. At 8C to 10C percentage of larvae detaching from artificial substrates was lower than 20%.