Between 1974-80, 12 municipal mosquito abatement programs at five locations across Ontario were monitored for larval mosquito survival and persistence of temephos. Breeding pools were checked for the production of spring Aedes species in April and early May and one site for Culex and summer Aedes in July. Where larval numbers exceeded 30 per 0.57 liter in the second or third instar, treatment with temephos was carried out. In those localities where less than 100 ha were involved, ground application was carried out; where areas to be treated exceeded 1000 ha, a helicopter was employed. Control in most cases was 100%. Initial residuals of temephos in water ranged from 1 to 220 ug/liter and declined rapidly over the first 24 hr period. Residues in sediments increased over the first 24 hr to levels between 0.1 and 11 mg/kg and then declined over the next 7 to 10 days. The current methods of applying temephos as a mosquito larvicide by trained and licensed personnel, resulted in residue levels in mosquito breeding habitats that were biologically active and environmentally acceptable within the local mandate for safe, effective mosquito control.