The K10 female sterility locus involved in establishment of the embryonic dorsoventral axis maps genetically to the 2E2-2F1 interval of the Drosophila X chromosome. We microdissected the 2E2-2F3 region from salivary gland chromosomes and used clones obtained from the microdissected fragments to establish a chromosomal walk covering more than 200 kb. To identify the K10 gene we used P-mediated transformation with cosmid clones constructed in cos-P, a cosmid vector incorporating the terminal repeats of the P element. Clone cos9, containing a 43 kb insert, transformed the germ line of homozygous K10 females and allowed production of normal progeny. It also rescued three genes, crooked neck, pecanex, and kurz, which map genetically near K10. Transformation experiments using smaller fragments of cos9 localize the K10+ function within 11 kb. Northern blots hybridized with probes from this region indicate the presence of several mRNA species. Each transcript has been assigned to a complementation group.