Laboratory bioassays were used to determine the efficacy of (S)-methoprene against 7 species of Australian mosquitoes. The 90% lethal concentration (LC90) ranged from 0.17 ppb for Aedes vigilax to 6.54 ppb for Culex sitiens. The survival of adults exposed as larvae to 2 sublethal dosages of (S)-methoprene was compared to a control group. Little effect was noted for Cx. sitiens and Culex annulirostris. However, survival of male and female Ae. vigilax was significantly reduced and appeared to be dose related. Bloodfeeding success of female Ae. vigilax was also significantly reduced.