Dow ET-57 in a bolus and as an additive in a feed formulation was tested in the fall of 1956 against cattle grubs, Hypoderma spp. Four hundred seventy-nine Hereford steers and heifers were used, of which 235 head were located at the Fort Hays Branch Agricultural Experiment Station, Hays, Kansas, and 244 head were located at the Gooch Mills Juniata Experimental Ranch, Manhattan, Kansas. All treated animals were given 110 mg. of Dow ET-57/kg. of body weight. There were no consistent differences in gains in weight of the treated and untreated animals. Grub counts were made monthly or every 28 days in accordance with the cooperator’s weighing schedule and showed a significant decrease in the number of grubs in the treated animals. At the Fort Hays Branch station, 118 animals were treated with boluses and showed an average increase of 0.5 grub per animal. The 117 untreated animals showed an average increase of 11 grubs per animal. At the Juniata ranch, 80 head were bolused and showed an average increase of 0.38 grub per animal. Another 80 animals were treated by adding the insecticide to the feed and showed an average increase of 0.25 grub per animal. The 84 untreated animals showed an average increase of 3.2 grubs per animal. Blood cholinesterase investigations were conducted on 30 animals, 10 of which were treated with boluses, 10 were treated by feed and 10 were used as controls, and showed that Dow ET-57 produced a marked depression of enzyme activity. Toxic symptoms were not observed in any of the experimental animals.