Spraying all the cattle in a herd with 0.2% DDT suspension gave better protection from horn fly [Siphona irritans] attack than was the case where half the cattle in a herd was sprayed, although the two methods were equal following the first two applications. Dipping was less effective. Per pound of DDT applied, 3 sprayed herds gained 1202, 1285, and 2306 lb. each and one dipped herd 430 lb. more than comparable untreated herds. Calves gained 96, cows 46, and mixed herds 30 lb. per head more than in check herds. Stomoxys calcitrans and Musca domestica were satisfactorily controlled by spraying barns and sheds.