Ear tags containing 8% abamectin and 20% piperonyl butoxide were applied to pastured cattle to evaluate efficacy against southern cattle ticks, Rhipicephalus (Boophilus) microplus (Canestrini), during a 40-wk field study. Temperature fluctuations had an important impact on effectiveness of the tags. The first 19 wks, during spring and early summer (March-June) temperatures were optimum for tick survival, producing high tick numbers on treated animals (9 - 63 ticks) which were not different from untreated animals. During this time tags provided ≤ 53.7% control. By contrast, during the summer through midfall (July-October), high temperatures caused dramatic natural declines in the tick population, but tick numbers on treated animals were always lower (0 - 3 ticks) than untreated animals (4 - 16 ticks). Control during this period was 72 - 100%, indicating tags had a negative impact on tick survival beyond the natural attrition caused by high temperatures. During the last 9 wks (midOctober to midDecember), temperatures were again highly conducive to tick survival and tick numbers on untreated animals rebounded to previous levels (7 - 56 ticks), whereas treated animals produced significantly fewer ticks (1 - 9 ticks), resulting in 73 - 98% control. Results demonstrated that timing of ear tag application was critical to the expected efficacy. Tags applied in spring through early summer would likely provide low level control, whereas tags applied in summer through early fall would likely provide high level control. Additionally, tag application in summer through fall months would likely prevent the enormous buildup of ticks that would otherwise occur the following spring with no tag treatment.