Activities of yearling steers that were treated or not treated for control of Haematobia irritans (L.) were recorded from direct observation. During the day, untreated steers spent more time walking and resting than did treated steers, and it appeared that they traveled ca. 0.5 km/day more than treated steers. Untreated steers ruminated longer and masticated more boluses during the day than did treated steers, but they masticated each bolus fewer times and for shorter periods than did treated steers. Frequency of tail switching was always greater for untreated than for treated steers. Steer activity changes due to horn flies may account for the reduced weight gains attributed to this pest.