Behavioral responses of physiologically resistant and susceptible populations of the horn fly.Haematobia irritans(L.), were characterized. Dose–response studies demonstrated that behavioral resistance can take the form of hypersensitivity or a lowered sensitivity threshold, or both. The physiologically resistant population was resistant to permethrin, fenvalerate, deItamethrin, cypermethrin, and DDT. It was behaviorally resistant to fenvalerate and permethrin. Deltamethrin evoked a hyperirritable response, which was due either to behavioral cross-resistance or a previously unconsidered form of physiologically potentiated behavioral resistance. Responses to DDT and cypermethrin were not different in the resistant and susceptible populations. Studies using pyrethroids synergized with piperonyl butoxide indicated that behavioral resistance to the pyrethroids was due to evolution of a qualitatively different mechanism than that which accounts for irritation/repellence upon initial contact with an intrinsically irritating pyrethroid.