Efficacy of an organophosphate (OP) mixture acaricide, Ravap^® (Bayer, Shawnee, KS) was evaluated as a spray at 0.15 and 0.3% active ingredient (AI) on cattle infested with all parasitic stages of OP-resistant Rhipicephalus (Boophilus) microplus (Canestrini). Laboratory bioassays showed ticks were 18.7X more resistant to OP acaricides than a susceptible reference strain. Overall results demonstrated both concentrations produced significantly greater adverse effects on ticks in every measured parameter than were obtained from untreated ticks, except for female engorgement weight. Overall percentage control at 0.15 and 0.3% AI was 85.3 and 87.6%, respectively. Ravap was most effective against ticks treated in the larval stage and least effective against ticks treated in the adult stage. At 0.15 and 0.3% AI, control against adults was 79.8 and 76.2%, respectively, whereas control against ticks in the larval stage was 96.5 and 97.7%, respectively, with no significant differences. Control against ticks treated in the nymphal stage was intermediate (82.5% at 0.15% AI and 93.1% at 0.3% AI) and there was a significant difference between concentrations. Although this OP mixture acaricide provided good control against a highly OP-resistant strain of ticks, the control was still well below the 99% level required for use in the U.S. Cattle Fever Tick Eradication Program. Therefore, a single treatment with this mixture acaricide against OP-resistant ticks would still pose a risk of dispersing cattle harboring viable ticks to uninfested areas. Effect of pesticide application method (spray versus dip) and potential for Ravap use in an emergency tick outbreak situation are discussed.