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Effects of six organophosphorus compounds, viz., malathion, fenitrothion, fenthion, dichlorvos, temephos and chlorpyrifos were evaluated on larvae of Culax quinquefasciatus, the vector of lymphatic filariasis. Different diagnostic concentrations supplied by WHO were used and per cent mortalities determined. LC50 values, as determined by probit analysis, were 1.273, 0.1091, 0.0947, 0.0458, 0.0076 and 0.0038 mg L-1, respectively. Chlorpyrifos and temephos were the most toxic where 100% mortality was observed while fenitrothion, fenthion and dichlorvos were found in between these two extremes. Regression coefficient (slope), fiducial limits to LC50 and LC95 and heterogeneity of the response (x(2)) were calculated for each insecticide.