BACKGROUND: Indoor residual spraying (IRS) is widely used for malaria transmission control in sub-Saharan Africa. Resistance to pyrethroids in the mosquito Anopheles gambiae is a growing problem. There is an urgent need to develop long-lasting alternative insecticides to reduce selection pressure for pyrethroid resistance and to provide control with a single IRS application in countries with long transmission seasons. METHODS: Two capsule suspension formulations (CS) of the organophosphate pirimiphos methyl were evaluated as IRS treatments in experimental huts in an area of Benin where the mosquitoes Anopheles gambiae and Culex quinquefasciatus are resistant to pyrethroids but susceptible to organophosphates. The CS formulations were tested alongside an emulsifiable concentrate (EC) formulation of pirimiphos methyl and a CS formulation of the pyrethroid lambdacyhalothrin. RESULTS: The two CS formulations of pirimiphos methyl gave prolonged control of An. gambiae and Cx. quinquefasciatus. In cement huts application rates of 0.5 g/m(2) induced high mortality of An. gambiae for almost a year: overall mortality rates 87% (95% CI 82-91%) and 92% (95% CI 88-94%). In mud huts application rates of 1 g/m(2) induced high mortality of An. gambiae for 10 months: overall mortality rates 75% (95% CI 69-81%) and 76% (95% CI 68-83%). The EC formulation of pirimiphos methyl failed to control An. gambiae two months after spraying. The pyrethroid lambdacyhalothrin demonstrated prolonged residual activity in bioassay tests but failed to control pyrethroid resistant An. gambiae that entered the huts. Pirimiphos methyl CS was highly active against Culex quinquefasciatus and gave control for 10 months in cement huts and 6 months in mud huts. CONCLUSION: Pirimiphos methyl CS (Actellic 300 CS) applied at 1 g/m(2) shows great promise for providing prolonged control of pyrethroid-resistant An gambiae and for delaying pyrethroid resistance. An alternative to DDT, giving year-round transmission control in sub-Saharan Africa is now a realistic prospect.