A newly developed Haematobia spp. trap is described, and results are presented from field trials to reduce populations of adult horn fly, Haematobia irritans L., on 5 dairy farms in western Florida and Alabama during the summer of 1992. We compared fly infestations on milkers subjected to trapping, versus either dry cattle on the same farm or milkers on a nearby farm, without the trap but where traditional horn fly control practices were used. Results gave 96.9% (95% CI, 93.8-98.4) reduction compared with dry cattle with a mean count of 228 per animal, and 90.2% (84.5-94.5%) compared with milkers on the control farms with a mean count of 113. Trapping removed the need to use insecticides to control this pest on milking dairy cattle and so offers a practical, environmentally acceptable, safe, and sustainable means of horn fly control on cattle which pass through the trap regularly.