Fluorescent pigment-marked male and femaleMusca domestieaL., heterozygous for a reciprocal translocation between chromosome II, III, and V (that reduced their fertility to 32.5% in outcrosses), and for 2 recessive mutant marker genesbwb(brown body color-III) andoera(ocra eye color-V), were released into a native population for 7 generations at a pig-rearing installation near Gainesville, Florida. Males and females homozygous for the mutant markersbwbandoerawere also released. All released females had mated before release with one of the two types of males. The releases were startcd (July 6, 1970) after the native population had reached its initial seasonal peak. The mixed population, measured by grid counts and estimated to be 20,000 flies before the releases were started, remained at a fairly consistent density throughout the entire release program, even though an average of 1500–5000 marked flies were released at the site each day. The number of released flies recovered by netting never exceeded 1% of those released, and no mutant homozygotes (bwb; ocra) were ever recaptured. The low rate of recovery of translocation or mutant-bearing pupae collected near the release site indicated that most of the released flies migrated from the release area, probably because the incipient population had saturated the existing environment. For whatever reason, the number of translocation-bearing males and females that remained in the vicinity of the release site and that participated in the reproductive cycle was not large enough to cause a very significant reduction in fertility of the native females.