The optimum size range of particles of two carbamates and two organophosphorus compounds in deposits on plywood and plaster for contact toxicity to blood-fed females of Anopheles stephensi List, was 5–30 µm. The effect of particle size on the contact toxicity of these compounds of high intrinsic toxicity was not great and in this respect resembled that found previously for dieldrin and γBHC. In the few tests done, the concentration of active ingredient in wettable powder formulations was not an important factor in determining the contact toxicity of deposits of such toxic materials.