Abstract: The aim of this study was to describe the genetics of amitraz resistance evolution and to obtain the independent genes number involved. Three Mexican R. microplus tick strains were used, one susceptible to all acaricides and two amitraz-resistant. Larvae were reared on isolated naïve-heifers and maintained nine generations in laboratory conditions. From each generation and each strain, the amitraz LC50 was chosen as the selection concentration for each strain. The control population of susceptible R. microplus ticks was maintained throughout the study. Genetic estimators were obtained. Selection of nine generations produced a 29-fold increase in resistance. Estimated h2 to amitraz resistance were 0.3057, 0.4348, 0.4863 with effective number of factors (genes) 6.31, 5.93, 6.69 and the numbers of generations required for 10-fold increase in LC50 were 10, 8 and 4 for susceptible Media Joya, Palenque and San Alfonso amitraz-resistant strains respectively. The increase in resistance in all the three selected strains showed that a part of the variation in amitraz resistance in R. microplus is additive. It is suggested that insecticide resistance against amitraz is controlled by almost completely recessive more than one gene. But these differences may not necessarily translate to field performance of this acaricide.