The biological activity of pirimiphos-methyl (20% e.c.) and a chlorpyriphos-methyl/deltamethrin (20/1% e.c.) formulation applied at different concentrations to glass, jute, plywood and mud surfaees against Prostephanus truncatus (Horn) and Sitophilus zeamais (Motsch.), was compared over a period of 18 weeks. The bioassays were conducted at 28°C and 70% r.h., 1, 4, 7, 14 and 18 weeks after treatment. Insect mortality was significantly influenced by the type of surface, the insecticide concentration, the age of the insecticide deposit and the interactions between these factors. P. truncatus was more susceptible to chlorpyriphos-methyl/deltamethrln deposits than S. zeamais. Amongst the different surfaces, glass retained the highest persistence, followed by jute, plywood and mud against P. truncatus. The insecticide deposits on jute were the most effective in controlling S. zeamais.