Susceptibility of horn fly populations to insecticides was evaluated at the Tabuleiros Costeiros region in the states of Alagoas, Bahia, and Sergipe from April 2001 to December 2002. A total of 37 bioassays were conducted in 23 ranches from 12 municipalities. Fly susceptibility was determined by the impregnated filter paper method using the active ingredients diazinon, permethrin, and cypermethrin. A high susceptibility to diazinon was found, with LC50 varying from 0.23 to 1.41 microg/cm2 and resistance ratios (RR) = 1.3. On the other hand, resistance to pyrethroids was detected in all studied populations. Cypermethrin LC50 ranged from 19.94 to 1,928 microg/cm2 and RR from 21.2 to 4,484.4. Although resistance levels to permethrin were much lower (RR<6), all LC50 were significantly higher than the LC50 obtained from the susceptible population. All ranches relied on pyrethroids for horn fly control, most cypermethrin (88.46%) and deltamethrin (80.77%) products. Spraying, by using manual backpack sprayers, was routinely used in all ranches. History of insecticide use as well as inadequate control practices help to explain the spread resistance of horn fly populations to pyrethroids detected in the region.