Grub control in cattle given Dow ET-57 (Trolene4) and Dow ET-57 with purified phenothiazine during different months after heel fly activity ranged from 80 to 100%. There were no appreciable differences in the control obtained with dosages of 75 and 100 mg./kg. of Dow ET-57 or with 75 mg./kg. Dow ET-57 with 100 mg./kg, purified phenothiazine. Also, there were no apparent differences between the grub control obtained from treating cattle as early as May and those treated at anytime during the subsequent months prior to appearance of grubs on the backs.