A mutator, mu2, in Drosophila melanogaster has been identified recently that potentiates the recovery of terminal deficiencies. The deleted chromosomes behave as if they had been capped; that is, they are protected from degradation and from fusion with other chromosome fragments. The mutator maps near the telomere on the left arm of chromosome 3. Using the selectable marker Aprt, 150 deficiencies for region 62 of the cytological map have been recovered. These deficiencies identify the map position of mu2 as 62B11-C1. A yeast artificial chromosome spanning this region has been subcloned into lambda phage, and the positions of deficiency breakpoints on either side of the mu2 gene have been identified within the subclones. These positions limit the location of the left end of the gene to a 23 kb region. In the course of these experiments, three additional, presumptive mutant alleles were identified, suggesting that other mutator alleles remain undiscovered in many standard laboratory stocks.