Resistance of horn fly, Haematobia irritans (L.), to fenvalerate and permethrin was evaluated in Manitoba from 1987 to 1989. The number of resistant populations and the intensity of resistance increased during the period. In 1987, resistant flies were observed on three of 18 herds sampled. Resistance, determined by comparison with a susceptible strain at the LC50, was less than 6-fold for fenvalerate and 3-fold for permethrin except for one population whose resistance was 14-fold for permethrin. In 1988, resistant flies were observed on six of 26 herds sampled. Resistance ranged from 0.03- to 38-fold for fenvalerate and 0.1- to over 100-fold for permethrin. In 1989, resistant flies were found on 10 of 22 herds sampled. Resistance ranged from 1- to 62-fold for fenvalerate and 0.8- to over 100-fold for permethrin. Resistant populations were interspersed among susceptible ones.