Haematobia irritans(L.), sterilized by irradiation with80Co, were released into a semi-isolated population in W Texas to determine the effect on the natural population. The initial releases (ratio of 10 sterile: 1 fertile insect based on prerelease data concerning adult emergence) caused a downward trend in reproduction. Continued releases caused a decrease in reproduction in excess of 98% during the last 3 weeks of the 16-week study, and control exceeded 70% for 10 weeks. However, when the sterile: fertile ratio dropped below 6: 1, the decrease in reproduction dropped below 90%.