Drosophila melanogaster females homozygous forflex, an X-linked recessive mutation, do not survive. Hemizygous males are unaffected. Homozygous embryos appear to lack SXL, the product of theSex-lethal (Sxl) gene, apparently as a result of disruption ofSxl splicing. It is known that bothSxl and its somatic splicing regulators [snf andfl(2)d] also function in the development of the female germ line. For this reason, we investigated the role offlex in the germ line by generatingflex/flex clones inflex/+ females. Females carrying such clones in their germ lines do not lay eggs whereas females carryingflex+ eggs lay viable eggs. Additionally, DAPI staining of ovarioles showed that diploid germ cells that are homozygous mutant forflex do not complete oogenesis. These results indicate that theflex+ gene product may be required for the development of the female germ line.