The residual effect of temephos sand granules (1%) was determined using water samples obtained from drums located at Chaguanas and D'Abadie, Trinidad, W.I. The weekly bioassay against Aedes aegypti larvae showed that at Chaguanas and at D'Abadie over 70% mortality was observed after 4 and 6 weeks respectively. Laboratory experiments showed that with the complete turnover of water in drums, there was sufficient active ingredient to provide 70% control up to 6 days. The reasons for this performance of temephos are discussed.