Resistance to several major classes of acaricides, including amitraz, has been reported in Boophilus microplus in countries, such as Mexico and Brazil, where tick control relies heavily on chemical acaricides. Information on the genetic basis of resistance to a particular acaricide would provide an understanding of evolution of resistance in the field, and assist in designing new control strategies. We conducted a laboratory study to select for amitraz resistance in the Santa Luiza strain originated in Brazil. This tick strain demonstrated resistance to multiple acaricides, including amitraz, at the time of collection from the field. Laboratory selections with amitraz resulted in a significant increase in amitraz resistance ratio. A 188-fold resistance was achieved after several generations of selection. We conducted a study in 2003 to elucidate the mode of inheritance of resistance to amitraz in this tick strain. Reciprocal cross mating experiments were conducted using the resistant Santa Luiza strain and the susceptible Munoz strain to generate the F1 hybrids. The F1 hybrids were then backcrossed with the resistant parent strain. The F1 were also inbred to generate F2. The FAO larval packet test was used to determine the LC50s of amitraz and permethrin in larvae of the parent strains, the F1 hybrids, the backcrosses, and the F2 generation. The results indicated that resistance to amitraz was inherited as an incomplete recessive trait, and multiple genes may have contributed to the amitraz resistance observed in the Santa Luiza strain. There was also a significant maternal effect on the expression of amitraz resistance in the hybrids.