Ten animal systemic insecticides and thiabendazole were administered orally, intramuscularly, or as pour-on treatments to sheep, and nasal chambers of sheep were examined to determine effects on larvae of Oestrus ovis L., the sheep botfly. Treatments completely or highly effective against larvae of all three instars were: Bayer 37341 (= Bayer 9017) (O,O-diethyl 0-[4-(methylthio)-3,5xylyl] phosphorothioate) at 40 mg per kg orally; Bayer 37342 (= Bayer 9018) (O,O-dimethyl 0-[4(methylthio)-3,5-xylyl] phosphorothioate) at 30 and 40 mg per kg orally; Famophos (= American Cyanamid 38023) (O-[p-(dimethylsulfamoyl)phenyl] O,O-dimethyl phosphorothioate) at 100 mg per kg orally; Ruelene? (4-tert-butyl-2-chlorophenyl methyl methylphosphoramidate) at 100 and 170 mg per kg orally, and 200 mg per kg as a pour-on treatment; and Stauffer R-3828 [S-(p-chlorophenyl) phenylmethyl O,O-diethyl phosphorodithioate] at 100 mg per kg orally. Other treatments either were ineffective or controlled larvae of only one or two instars. A number of animal systemic insecticides control larvae of the sheep nose botfly, Oestrus ovis L., in the nasal chambers of sheep (for a review see Bushland et al., 1963). Most of these insecticides were administered orally or intramuscularly to the animals. Recently Pfadt (1964), using lambs infested with firstand second-instar larvae only, reported the effectiveness of five insecticides administered not only orally or intramuscularly, but also as nasal ointments, nasal sprays, and/or pour-on