The effectiveness of two acaricides was evaluated for control of populations of the southern cattle tick, Boophilus microplus on dairy cattle in St. Croix, U.S. Virgin Islands. Two different concentrations of a microencapsulated (ME) formulation of permethrin (0.05% and 0.10% ai) and one concentration of coumaphos emulsifiable concentrate (EC) (0.117% ai) were sprayed on Holstein heifers that had a natural infestation of ticks. The length of residual activity of EC coumaphos and ME permethrin at 0.05% (ai) was at least four days, and for ME permethrin at 0.10% ai at least seven days.There was no apparent increase in the residual activity period of ME permethrin in comparison to that in field trials of other formulations of permethrin against B. microplus. Percentage control for the three treatment groups from days 4 through 21 was ME permethrin (0.05% ai) 96%; ME permethrin (0.10% ai), 97%; coumaphos, 98.3%. There were no significant differences among the treatment groups (F = 2.21, df = 2,17, P> 0.10). This research confirms the usefulness of permethrin as an alternative to coumaphos for control of B. microplus populations on cattle.