OBJECTIVE: To determine the efficacy of chlorfenapyr against Anopheles arabiensis and Culex quinquefasciatus in East Africa and to identify effective dosages for net treatment in comparison with the commonly used pyrethroid deltamethrin. METHODS: Chlorfenapyr was evaluated on bed nets in experimental huts against A. arabiensis and C. quinquefasciatus in Northern Tanzania, at application rates of 100-500 mg/m(2). RESULTS: In experimental huts, mortality rates in A. arabiensis were high (46.0-63.9%) for all dosages of chlorfenapyr and were similar to that of deltamethrin-treated nets. Mortality rates in C. quinquefasciatus were higher for chlorfenapyr than for deltamethrin. Despite a reputation for being slow acting, >90% of insecticide-induced mortality in laboratory tunnel tests and experimental huts occurred within 24 h, and the speed of killing was no slower than for deltamethrin-treated nets. CONCLUSIONS: Chlorfenapyr induced low irritability and knockdown, which explains the relatively small reduction in blood-feeding rate. Combining chlorfenapyr with a more excito-repellent pyrethroid on bed nets for improved personal protection, control of pyrethroid-resistant mosquitoes and pyrethroid resistance management would be advantageous.