Experiments were conducted to evaluate the response of 3rd-instar larvae of the horn fly,Haematobia irritans(L.), to laboratory-simulated bovine manure pat temperature gradients. Significant differences were found to exist between distributions of larvae in pats with uniform and gradational temperature profiles. Larvae exhibited a positive thermotaxis toward warmer, but nonlethal, temperatures in the upper strata of the pat from 0800 to 1000 h; however, between 1400 and 1800 h, a negative thermotactic response to potentially lethal temperatures occurred as larvae were recovered primarily from the comparatively cooler, lower strata of the pat.