Treatment of rice fields in Sri Lanka with monocrotophos at 10 mg/litre or pirimiphos-methyl at 0·1 mg/litre gave a selective advantage to larvae of Anopheles subpictus Grassi and A. nigerrimus Giles carrying the oxidase- and acetylcholinesterase-based resistance genes, respectively. However, this selective advantage was apparent for less than ten days after spraying. There was no mortality of any larval instar with monocrotophos 12 days after spraying, and no fourth-instar larval mortality with pirimiphos-methyl 17 days after spraying. The chemical degredation curves for these compounds indicated that this short duration of efficacy was due to the instability of the compounds in water under field conditions. In contrast, the new growth regulator S-31183 (2-[1-methy1-2-(4-phenoxyphenoxy)ethoxy]pyridine) sprayed at 0·1 mg/litre conferred no selective advantage to larvae with either resistance mechanism and had a total efficacy period of at least 71 days. This difference can be attributed to the greater toxicity of the growth regulator and its slower chemical degradation under field conditions when compared to the organophosphates.