Male and female house flies,Musca domesticaL., bearing a heterozygous translocation that reduced fertility to ca. 32.5% in outcrosses and ca. 15% in intracrosses were released at a poultry farm. Fertility of the mixed population at the release site (determined weekly) ranged from 9.1 to 32.0% during the first 9 wk of the test; fertility at the control sites ranged from 63.4–95.7%. Average density (from weekly grid counts) ranged from>1 to>4 at the release site (an avg 1400 flies were released/day), 0 to>2 at control site 1, and 0–20 at control site 2. A high ratio of translocation-bearing to native flies could not subsequently be maintained at the test site during the final 14 wk of the test even though a larvicide was applied there. However, as long as the released translocation-bearing flies were at a high ratio to the native flies, fertility was low, and laboratory-reared semi-sterile flies did participate in the reproductive cycle.