Tests of a synthetic insect growth regulator (Altosid SR-10, or Methoprene, Zoecon Corp., Palo Alto, Calif.) have been completed recently at Chalk River Nuclear Laboratories. This paper reports on the effectiveness of this promising material in tests on several species of mosquitoes of this area. The low toxicity to mammals, the relative absence of deleterious effects on non-target aquatic organisms under normal use (Staal 1975), the fact that Altosid is readily biodegradable and is effective at low rates of application would recommend this material for the control of mosquito larvae. Reports in the literature on the use of juvenile hormone-type compounds have dealt mainly with mosquitoes in the southern United States; for example, Schaefer and Wilder (1973) reported on Aedes nigromaculis, A. melanimon, and Culex tarsalis in California. In our tests, we wished to determine its effectiveness on local species and in mosquito breeding pools where temperatures would be lower than in the work mentioned above.