The male sterile gene, tw2, has been suggested as a possible means for controlling population size of feral mice because of its detrimental effects on fertility in homozygous males and because of its effects on transmission ratios in heterozygous males . This suggestion was tested by releasing mice carrying the tw2 gene into a population of mice, known to be free of t alleles, housed in a large outdoor enclosure. Attempts to establish +/tw2 males were unsuccessful, but +/tw2 and tw2/tw2 females bred successfully when introduced into an empty territory at the beginning of the breeding season. Following successful introduction of the gene, the size of the enclosure population increased fairly steadily for 18 mth, and a sample of males removed from the enclosure 2 yr after the successful release was found to be tw2-free. It was concluded that, under the conditions used in this experiment, the gene had little or no effect on population size and that it probably disappeared fairly rapidly from the population.