Mutations I48L, I54V, R86Q, V137I, I492M, and T548A were identified previously in BmAChE3, a gene encoding acetylcholinesterase, from the organophosphate (OP) acaricide-resistant San Rommn strain of Rhipicephalus (Boophilus) microplus. Recombinant BmAChE3 acetylcholinesterase containing the R86Q mutation was shown to exhibit nearly 20-fold reduction in the rate of phosphorylation by paraoxon relative to the wild-type sequence. In addition, the R86Q mutation was present in resistant laboratory strains at elevated frequency compared with OP-susceptible strains but was insufficient to alone generate the OP-resistant phenotype (J. Med. Entomol. 44: 1013-1018). Here, we developed assays to genotype the remaining five mutations and evaluated frequency of all six BmAChE3 mutations in individual R. microplus ticks from laboratory and Mexican field-collected strains. We found a substantial number of individuals in known OP-susceptible strains that seemed to be homozygous for each of the mutations surveyed, the exception being I48L, which was infrequent in all strains, leading us to conclude that none of the mutations alone were responsible for generation of phenotypic resistance to OP acaricide.