Horn fly, Haematobia irritans (L.), populations in northeastern Mexico were surveyed for resistance to fenvalerate and coumaphos. Compared with a susceptible strain of horn flies, resistance levels among field populations for the pyrethroid, fenvalerate, ranged from 36 to 199 times at the LD50 level, indicating that horn fly control with pyrethroids would be unsatisfactory. Resistance to the organophosphate compound, coumaphos, was less evident, with flies at only 1 of 5 locations showing levels of tolerance that might interfere with adequate control.