The insect growth regulators methoprene, Thompson-Hayward TH 6040 (N-(4-chlorophenyl)-N'-(2,6-difluorobenzoyl)urea), and Hoffmann-La Roche Ro 20-3600 ((E)-4-[(6,7-epoxy-3,7-dimethyl-2-nonenyl)oxy]-1,2( methylenedioxy) benzene), were fed to Holstein cows in amounts sufficient to control larvaeMusca autumnalisDe Geer in their feces. Methoprene was not active against nontarget insects, Ro 20-3600 was active against Hydrophilidae and Scarabaeidae, and TH 6040 was active against all the major nontarget insects except Scarabaeidae.