Abstract Ten spot-on or pour-on treatments of systemic insecticides were applied on October 12, 1977, to lambs for control of the larval stage of the sheep bot fly. Lambs were randomly assigned to each treatment and to one control group. After 7 to 8 days, the sheep were sacrificed and the nasal cavities and sinuses examinee for live larvae. Larvae were mainly first instar, but a small % were second instar.