BACKGROUND & OBJECTIVES The resistance in vectors to conventional insecticides warrants development of newer insecticides for mosquito control besides use of other counter-measures. The present study was undertaken to test the efficacy of a new and toxicologically safe organophosphorus compound, Reldan (chlorpyrifos-methyl) and compare its efficacy with Dursban (chlorphyrifos-ethyl) against larvae of different mosquito vectors under laboratory conditions and against Culex quinquefasciatus in three larval habitats. METHODS The susceptibility of larvae of mosquito vector species to Reldan and Dursban was determined by following the standard procedure and the LC50 in mg/l values were calculated by probit analysis. The insecticides were evaluated in the larval habitats of C. quinquefasciatus in and around Pondicherry. The percentage reduction in larval density was calculated using Mulla's formula and duration showing more than 80 per cent reduction was calculated by annotating the graph obtained by plotting percentage reduction in larval density against days of post-treatment. RESULTS Dursban and Reldan were effective against Anopheles fluviatilis larvae at lower LC50 of 5.90 x 10(-7) and 1.07 x 10(-9) mg/l respectively compared to other species of mosquitoes. Dursban and Reldan were equally effective against Aedes aegypti, An. stephensi and An. culicifacies. Reldan was about nine times more effective than Dursban against C. quinquefasciatus with respective LC50 of 1.17 x 10(-4) and 1.34 x 10(-3) mg/l. Field evaluation of these insecticides against C. quinquefasciatus showed that Reldan and Dursban at the application rate of 500 g (ai)/ha reduced immature density by more than 80 per cent for a mean duration of 4.0 and 4.5 days and 4.6 and 3.7 days in cesspits and cement tanks respectively. In U-drains, at 500 g (ai)/ha, Reldan was effective for a period of 3.6 days whereas, Dursban was not effective. It was effective for 4.0 days only at 1000 g(ai)/ha. There was no significant increase in the residual activity of both Dursban and Reldan with increase in application rates. INTERPRETATION & CONCLUSION In view of the lack of sustained control with the applications, it is inferred that these insecticides will be of limited use as larvicides against C. quinquefasciatus breeding in polluted habitats.