Five different studies on the application of insecticides to cattle for horn fly ( Haematobia irritans (L.)) control were conducted in Mississippi during 1961. Residual pen sprays of 0.25% Sevin®(l-naphthyl N -methylcarbamate) wettable powder remained effective longer than the sprayable formulation, but even the wettable powder controlled flies for only 10 to 12 days. Bayer 29493 ( O, O -dimethyl O -[4-(methylthio)- m -tolyl] phosphorothioate) at 0.1% was effective about 10 days. In a comparison of pen spraying and back-rubber treatments with Shell SD4294 (dimethyl 2-( alpha -methylbenzyloxycarbonyl)-l-methylvinyl phosphate), 0.1% sprays required 7 applications, 0.25% sprays 4 or 5, and 2% back rubbers 2 applications to provide practical all-season horn fly control. Tests of a semi-automatic sprayer of original design indicated that the simple, inexpensive device might have practical use. In tests with photo actuated automatic sprayers, the application of synergized pyrethrins provided complete horn fly control all season. DDVP at 0.01% and Shell SD4294 at 1% also provided horn fly control when applied daily with the automatic sprayer. Animals having access to ronnel-medicated salt had lower horn fly populations than animals receiving plain salt. Feces of animals receiving 2 or more mg./kg./day of ronnel were toxic to larvae of house fly ( Musca domestica L.) and horn fly.