We report an efficient and specific gene targeting method for transforming the germ line of Drosophila melanogaster. The targeting occurs during the repair of a double-strand DNA break that is induced at the white locus by the excision of a P transposable element. The break is repaired when homologous sequence is copied from a plasmid injected into the Drosophila embryo. The procedure efficiently integrates DNA into the targeted locus of the Drosophila genome. Heterologous sequence of up to 13 kbp in length can be inserted, permitting the intergration of entire genes into a common genomic site for further study.