Abstract Plastic ear tags containing 13.7% wt/wt RabonR (stirofos) were affixed to both ears of cattle in 2 locations in Missorui. A total of 43 head in 2 herds was treated in Linn Co., plus a single herd of 60 head in Boone Co. In the former location, an additional herd of 50 animals was treated on June 29 by hanging a new device from each ear. This apparatus, patented by Mr. Roy Goodwin (Lancaster, Mo.) consisted of a small injection-molded plastic bottle with a ring on the top. A threaded neck on the bottom provided attachment for a plastic screw-cap. A 0.5 cm hole in the latter permitted egress of a 22 cm cotton wick. The entire device was held to the ear via a flexible plastic strap passed through the ring, doubled, and inserted through a hole augered in the ear between 2 ribs of cartilage. A molded cap pressed over the plastic strap secured the device against loss. Each device was charged with 20 ml of 140 wt gear oil containing 1% dichlorvos (v/v). In addition, the wick was soaked in this solution prior to attachment. Test animals received these devices in both ears. Assessment of treatment efficacy was made immediately prior to treatment and threrafter at weekly intervals via inspection of 10 head selected at random from each herd. Untreated herds located nearby served as controls and were inspected in a similar manner. Face flies were counted on the entire face, and horn flies on a single side of each animal examined.