Bioassay studies reveal the toxicity levels of pesticides utilised in the area to the larvivorous fishesAplocheilus lineatus andMacropodus cupanus. The resistance of both fishes decreases with increase in period of exposure to the pesticides. Comparing the major groups of synthetic organic pesticides, the chlorinated hydrocarbons, here exemplified by DDT, are more toxic to the fishes than ekalux and malathion, the organophosphates experimented with. The carbamate sevin is the least toxic. Nevertheless, all the pesticides are ‘toxic’ to ‘very toxic’ as defined by the Joint ICMO/FAO/UNESCO/WHO group of experts, having an acute lethal threshold of below 1 to 100 mg/l.M. cupanus is the more resistant of the two fishes, probably on account of its obligate air-breathing nature, and thus its tendency to absorb less toxicant across the gills. Contrasting the susceptibility of mosquito larvae and the fishes studied to the pesticides investigated, the closeness of the LC50 values obtained inA. lineatus to that recorded in certain species of mosquito larvae indicates that whileM. cupanus could be employed in conjunction with pesticides for anti-larval work,A. lineatus should not be so utilised.