The ovo locus is required for the maintenance of the female germ line in Drosophila melanogaster. In the absence of an ovo gene, males are completely normal but females have no germ-line stem cells. Three dominant mutations at the ovo locus, called ovo, were observed to revert towards recessive alleles at high frequency when ovo males were crossed to females of the strain y v f mal. We have found that this strain contains an inordinately high number of gypsy transposable elements, and crossing it with the ovo strains results in the mobilization of both gypsy and copia, with high-frequency insertions into the ovo locus: of 16 revertants examined 12 have gypsy and four have copia inserted at 4E, the ovo cytological site. Using gypsy DNA as a tag we have cloned 32 kb of wild-type DNA sequences surrounding a gypsy insertion and characterized molecular rearrangements in several independent revertants: in 10 of them gypsy appears to be inserted into the same site. The orientation of gypsy is strictly correlated with whether the neighbouring lozenge-like mutation appears in the revertants. A distal limit of the ovo locus was molecularly determined from the breakpoint of a deletion affecting closely flanking regions.