Field developmental time of the horn fly,Haematobia irritans(L.), was investigated for 3 years in Missouri, and the data were analyzed by determining the least-squares regression between mean temperature during development and the developmental rate. The pooled predictive equation was calculated to be: 1/T = −0.04769 + 0.005168 θ; T = time required for development (in days) and θ = mean temperature (°C) during development.