The temporal, climatic, and physiological factors mediating dispersal of a naturally occurring population of horn flies,Haematobia irritans(L.), were examined. Resident (nondispersing) horn fly populations increased from late spring through early fall and decreased in late October. Fly-free herds 0.4 km from the source herd were reinfested from the source population at a significantly slower rate in June (4.6 flies per side per animal per day) than in August or October (7.1 flies per side per animal per day). Given that the greatest increase in the resident population occurred in June, it would appear that horn fly dispersal is not density-dependent. However, the rate of reinfestation was greatest in cattle treated with a larvicidal bolus and a daily application of a short-residual insecticide, indicating that reinfestation may be a function of competition for hosts, with the most rapid colonization occurring when few horn flies are present. The rate of reinfestation did not differ as a consequence of bolus treatment, suggesting that newly emerged flies rapidly disperse or replace older, dispersing adults. Physiological age-grading showed that the age structure of the dispersing flies did not differ significantly from that of the resident population. Moreover, all age classes were included in the dispersing population, with the largest proportion being parous adults. The dispersing population was primarily female, which emerged earlier and in greater numbers than males.