In this study, acetylcholinesterases (AChEs) were extracted from two Mexican Boophilus microplus strains that demonstrated resistance to the organophosphate (OP) acaricide, coumaphos, in bioassay. The rate of inhibition of the extracted AChEs by the diethyl-OP paraoxon was determined for two resistant strains and two susceptible strains of B. microplus. The time to inhibition of 50% AChE activity was approximately two-fold greater for the resistant strains. Kinetic analysis of the interaction of the resistant AChEs with paraoxon revealed reduced bimolecular reaction constants (k_i). Apparent conformational changes in the AChE of the resistant strains were reflected in reduced K_m and V_max values. The bimolecular reaction constants (k_i) of the resistant strains were most affected by a slower rate of enzyme phosphorylation (k₂).