ABSTRACT Chemical acaricides are used as the primary method for controlling Rhipicephalus (Boophilus) microplus. However, the incorrect use of these compounds over the years has led to the emergence of resistant tick populations. This study aimed to evaluate the effectiveness of different combinations of synthetic acaricides and the essential oil of Plectranthus amboinicus on engorged females of R. (B.) microplus. Engorged females were obtained from infested bovines in dairy farms in the Agreste mesoregion of Pernambuco, Brazil. Five different combinations of commercial synthetic acaricides: I) Chlorpyrifos 30.0g + Cypermethrin 15.0g + Fenthion 15.0g; II) Chlorpyrifos 8.5g + Cypermethrin 15.0g + Ethion 16.0g; III) Deltamethrin 50.0g; IV) Amitraz 12.5g; V) Chlorpyrifos 15.0g + Cypermethrin 25.0g + Piperonyl butoxide 50.0g, and the 1% essential oil of P. amboinicus were used for the Adult Immersion Test. Only the combinations I and V of synthetic acaricides and essential oil showed efficacy over 95%. The findings of this study demonstrate that ticks’ resistant populations are present in the study area, and the essential oil of P. amboinicus is a promising compound for developing products used for tick control. Finally, it is paramount to implement educational activities to improve farmers' awareness of the rational use of acaricides.