Dispersal of wild horn flies, Haematobia irritans (L.), from existing populations to fly-free cattle was studied in Louisiana and Kansas. Rates at which horn fliesinfested fly-free cattle placed sequentially, for 10-d intervals, at 0.05, 0.8, and 1.7 km from a source population, did not differ significantly and averaged 18 flies per side per animal per d. Regardless of distance from the source, fly-free cattle were infested with pest populations as great as those present on the source herd within 6 d. For a 13-d period, the rate at which horn flies infested herds placed simultaneously at 0.05 and 0.8 km from a source population did not differ significantly, indicating that the nearer population did not function to trap flies dispersing from the source population. A herd placed behind a border (0.5 km) of trees, 1.7 km from the source, showed a significantly decreased rate of infestation of five fliesper side per animal per day. However, combined factors of long distances, trap animals, and physical barriers did not prevent infestation of cattle by dispersing horn flies.