MaleMusca domesticaL. bearing a heterozygous chromosomal translocation which reduced fertility to 32.5% of control were released at a poultry house in a pine forest near Gainesville, Florida, during the summer of 1969 to study methods of release and to determine whether the released flies would introduce the translocation into the native population. This represents the first field test of the concept of genetic manipulation in a natural population of insects where male insects that possessed the same induced genetic aberration (induced and recovered in one insect and subsequently multiplied) were released to decrease the natural level of fertility. Also, a laboratory simulation of the outdoor release was conducted in a 19-cubic-meter room. When the percentage fertility was based on the number of pupae obtained from a given number of eggs, control fertility (measured in an outdoor screened-in chicken coop containing a strain of native house flies) was 78.2% for the 4-generation test period; fertility at the forest release site was 63.5%; and fertility in the laboratory simulation was 27.7%, a result much closer to theoretical expectations than that obtained at the forest site. Thus, the test had a limited success and raised some questions, but it provided important information.