The weight gains of yearling calves were analysed in a field experiment primarily designed to compare the distribution of flies among animals in two large herds, one unprotected and the other treated regularly for the control of biting flies. The treated animals received three applications of Co-Ral as a 0.5% wettable-powder suspension at 3-week intervals. Weight gains in the treated herd were lower by 6.5% of body weight as compared with the control herd. Weight gains increased in the treated herd after DDT was substituted for Co-Ral in the fly-control operation. The reduction in weight gains in the treated herd was attributed to subclinical toxicity of the organophosphate pesticide during repeated treatments.