Technical Abstract: Pastured cattle with ear tags containing 8% abamectin synergized with 20% piperonil butoxide were evaluated for efficacy against Rhipicephalus microplus ticks during a 40-week study conducted under natural South Texas conditions. In the first 16 weeks, which occurred during the spring and early summer months when tick survival was optimum, the control was indistinguishable in comparison to untreated cattle. Subsequently, between Week 17 and Week 24 post-treatment when temperatures were near maximum, a natural decline in the tick population occurred in both the treated and untreated pasture, although fewer ticks were observed on treated cattle than on untreated cattle. This indicated tags had an adverse affect on tick survival beyond the adverse affect caused by climatic conditions. Between Week 25 and Week 40 when temperatures again decreased to moderate levels in the fall and winter months, tick numbers on untreated cattle rebounded toward previous higher levels, whereas tick numbers on treated animals remained very low. While eradication of the ticks was not expected, nor was it an objective of the study, results demonstrated that substantial suppression of ticks could be obtained if tags were applied at the proper time. Initial application of tags in late winter through early summer when tick numbers are near maximum, would have little observable effect on the population in the short term. Conversely, application of tags in late summer through early fall when tick numbers are naturally low would prevent buildup of ticks that normally occurs in the spring by eliminating many ticks that attain a host in fall and winter months.