During the past two decades, resistance to pyrethroids within the cattle tick genus Boophilus has caused tick control problems in various tropical countries, mainly in Latin America, southern Africa, Australia, and New Caledonia. A Rhipicephalus (Boophilus) microplus (Canestrini) strain from Costa Rica, exhibiting resistance to the pyrethroid deltamethrin but only a very low resistance to organophosphates (OP) was kept under selection pressure for 9 to 11 generations by using deltamethrin or coumaphos (OP), either exclusively or in rotation. The objective of this acaricide rotation was to examine the possibility of delaying or reducing the full emergence of pyrethroid resistance. In the substrain selected with deltamethrin at the LD50 concentration, resistance to deltamethrin was measured after five generations (resistance factor [RF] = 9.2) and very high resistance after 11 generations (RF = 756), compared with the starting field strain from Costa Rica. In the substrain selected with deltamethrin then coumaphos in rotation, resistance to deltamethrin was variable from one generation to the next (RF = 1-4.2), but no high, stable resistance developed. After 10 generations of rotation, the deltamethrin RF was 1.6. In the substrains selected continuously with coumaphos or coumaphos and deltamethrin in rotation, no consistent change in resistance to coumaphos was observed. Rotation of deltamethrin with coumaphos seems to delay the development of strong resistance to deltamethrin in a population that had initially a low level of deltamethrin resistance.