Abstract Background Determining the insecticide resistance status of malaria vectors, particularly to insecticides used on mosquito nets, is important but is limited to a relatively small number of locations. We describe a simple assay that enables this information to be obtained over a much wider area. Methods The time to knockdown of mosquitoes in an insecticide-treated netting-covered metal frame cage were recorded. The shape of the curve of the proportion of knocked down mosquitoes provides information on resistance status. Results Resistant Anopheles funestus took significantly longer for knockdown than did susceptible Anopheles arabiensis. Conclusions This simple technique will enable a wide range of locations to be sampled, enhancing our understanding of the spread of resistance to pyrethroids.