Only a few Drosophila melanogaster germline sex determination genes are known, and there have been no systematic screens to identify new genes involved in this important biological process. The ovarian phenotypes produced by females mutant for dominant alleles of the ovo gene are modified in flies with altered doses of other loci involved in germline sex determination in Drosophila (Sex-lethal+, sans fille+ and ovarian tumor+). This observation constitutes the basis for a screen to identify additional genes required for proper establishment of germline sexual identity. We tested 300 deletions, which together cover approximately 58% of the euchromatic portion of the genome, for genetic interactions with ovoD. Hemizygosity for more than a dozen small regions show interactions that either partially suppress or enhance the ovarian phenotypes of females mutant for one or more of the three dominant ovo mutations. These regions probably contain genes whose products act in developmental hierarchies that include ovo+ protein.