The response of laboratory-reared and wild horn flies,Haematobia irritans(L.), to three temperature regimes was studied with an olfactometer. Response comparisons were made between average temperatures of 26 and 31°C (regime 1), 27 and 36°C (regime 2), and 31 and 36.5°C (regime 3). With 2.5 liters/min airflow and a light behind the test cage, 88–97% of older laboratory flies responded to the warmer side of the olfactometer with regimes 1 and 2, but with regime 3 only 66–68% responded to the warmer side. With the light in front, 70–76% responded to the warmer side with regime 2. Both sexes of young horn flies (6–15 h old) or newly emerged females (<6 h old) tested at 2.5 liters/min and with a light behind during regime 1 preferred (73–78%) the warmer side; 92% of newly emerged (<6 h old) male flies preferred the warmer side. With both sexes of wild flies of mixed ages 68–70% preferred the warmer side of the olfactometer. Flight activity of laboratory flies of all ages was less after heat application; wild fly activity was unchanged. Probing response to heat was greatest by newly emerged and young horn flies. Probing response of wild flies was slightly less than that of newly emerged or young flies.