 The proposed IRS intervention, Actellic CS Syngenta, is a microcapsule formulation of the organophosphate insecticide, pirimiphos-methyl. This product has been the subject of a 3 year public-private partnership between the company, LSHTM, the Bill and Melinda Gate BMG Foundation and the Innovative Vector Control Consortium IVCC. Microencapsulation greatly extends the compounds residual life when applied to interior walls and ceilings. Parallel trials in Benin against pyrethroid-resistant mosquitoes killed more than 80 of An. gambiae for up to 10 months, which is an unprecedented level of effectiveness. Other products tested at the same time included bendiocarb, the carbamate used in our last cluster randomised trial in Muleba, and lambdacyhalothrin, the pyrethroid used by PMI for IRS in Muleba during the previous 5 years. Each of these was vastly inferior or short lived lasting 1-2 months, making pirimiphos-methyl CS the best candidate to replace carbamate or pyrethroid IRS. 