In Colombia, the control of the cattle tick Rhipicephalus (Boophilus) micropluslies almost exclusively on the use of chemical acaricides. However, R.(B.) microplus has developed resistance to most of the commercially availableproducts. The aim of this study was to assess the degree of resistanceto the major topical acaricides used in Colombia. The adult immersiontest was used as recommended by FAO with ticks from 7 farms originatingin 4 distant Colombian regions. The efficacy of the products was calculatedbased on the reproductive parameters of fecundity and fertility withgroups of ticks (n = 40) exposed to the recommended concentrations ofeach acaricide, and compared with the respective control group exposedto distilled water. In general, products containing deltamethrin (25 ppm)and amitraz (208 ppm) were less effective, with efficacy values of <20%and 10-50%, respectively. Doubling the concentration did not increase theefficacy of either compound. The combination of chlorpyrifos (312 ppm) +cypermethrin (150 ppm) showed great variation between farms, with anefficacy ranging between 22-93%. Unlike chlorpyrifos, the organophosphorusethion (622 ppm) was 99-100% effective in completely inhibitingoviposition in 4 of the 7 farms. In conclusion, resistance to deltamethrinand amitraz is very high in seven distant strains of the Colombian territory,varies widely for chlorpyrifos, and was not detected for ethion that was99-100% effective.