A field study was undertaken to evaluate health risks in persons engaged in ultra-low-volume (ULV) aerial application of technical malathion for mosquito control. No specific toxicity signs and symptoms were observed except nausea and irritation of eyes in a few cases. Observations on the ChE activity has revealed a significant depression in plasma at the end of spray operation in labourers employed in loading and unloading of insecticide and in field workers. Red blood cells ChE activity was not affected in any group of subjects. At the end of spray operation the insecticide residue on the grass used as cattle feed were about 315 ppm, which are much above the permissible level. However, 4 days after the application normal levels of the insecticide residues were detected.