The emergence of strains of malaria vectors resistant to malathion in an area of Pakistan, and the continuing search for improved methods of control, necessitated the examination of alternative safe insecticides, with improved residual effects, for future use in the Malaria Control Programme in Pakistan. For these reasons, the effectiveness of pirimiphos-methyl, as Actellic 25 WP, was evaluated on a large scale in one sub-sector of Sheikhupura district of Punjab Province near Lahore. Entomological and parasitological evaluations demonstrated that 1 g of pirimiphos-methyl/m2 was as effective as 2 g/m2. Vector mosquito densities were reduced to zero, or almost so, in all areas sprayed with pirimiphos-methyl, and only began to approach vector levels in unsprayed areas after 9-10 months. No new cases of malaria were detected in those areas sprayed with pirimiphos-methyl. Blood cholinesterase determinations after the application of the pirimiphos-methyl spray confirmed the absence of any toxic effect on the spray operators, nor were there any toxic symptoms in the house occupants.