Acetylcholinesterase cDNAs, BmAChE1, BmAChE2, and BmAChE3 of Rhipicephalus (Boophilus) microplus (Canestrini) were sequenced and found to exhibit significant polymorphism. A portion of the predicted amino acid substitutions in BmAChE1, BmAChE2, and BmAChE3 were found predominantly in organophosphate-resistant strains, but most did not correlate with resistant status. Multiple transcripts were observed from individual ticks, suggesting possible gene duplication or alternative splicing to produce more than two transcripts per individual. BmAChE1 transcript polymorphisms associating with organophosphate-resistant status in laboratory strains were surveyed in laboratory and Mexican strains of R. microplus by sequencing BmAChE1 genomic DNA. Quantitative real-time polymerase chain reaction was used to determine copy numbers of BmAChE1 (eight copies/ haploid genome), BmAChE2 (16 copies/ haploid genome), and BmAChE3 (four copies/ haploid genome). Presence of at least three highly polymorphic amplified genes expressing AChE in tick synganglion suggested that ticks maintain a large and diverse assortment of AChE alleles available for rapid recombination and selection, which potentially reduces fitness costs associated with individual mutations. Elevated copy numbers for each of the BmAChEs may also explain previous failures to identify mutations resulting in insensitivity to organophosphates. It is clear that development of phenotypic resistance to organophosphates is highly complex and may be multigenic in character.