Boophilus microplus, collected from Starr County, Texas, were determined to be resistant to the organophosphorous acaricides coumaphos and diazinon. Initial bioassay results from collected ticks produced a slope (SE) of 3.96 (0.22) which was different from that obtained from a susceptible reference population 6.97 (0.38). Resistance ratios (RR) (95% C.I.) indicated that the population was resistant to coumaphos 3.6 (3.4-3.8), 5.0 (4.5-5.5), and 6.5 (5.4- 7.7) at the LC 50, 90 and 99, respectively. Bioassay results generated from a second collection of ticks made 12 d after all cattle in the infested pasture were treated with coumaphos, confirmed coumaphos resistance, but produced a slope (SE) that was not significantly different from a susceptible laboratory reference population. The ticks were also resistant to diazinon, RR (95% C.I.)=7.1 (6.5-7.7), 11.7 (10.3-13.3), 17.7 (14.5-21.5) at the LC50, 90, and 99, respectively. The slope (SE) generated from the diazinon bioassay was different than that of a reference strain, 2.98 (0.12) and 6.09 (0.35), respectively. The high dose strategy used by the Cattle Fever Tick Eradication Program was able to eradicate coumaphos resistant B. microplus after just two treatments of coumaphos, 12 d apart.