Amorphic mutations in the sex-determining gene Sex-lethal are female lethal, but a small set of hypomorphic point mutations ( Sxlf4, Sxlf5, and Sxlf18) disrupt only Sxl's germline functions. Females with these mutations are fully viable, but are sterile due to a block in oogenesis that generates ovarian germline tumors. Laboratory stock crosses produced a Sxlf4 line in which homozygous mutant females were weakly fertile. These females produced large numbers of eggs but few offspring, because the meiotic defects also associated with Sxlf4 were not rescued. Suppression of Sxlf4 female sterility proved to be maternally transmitted, leading us to consider Wolbachia, an intracellular bacterial parasite known to alter reproduction and sex determination in a variety of arthropods. We discovered that Sxlf4 suppression could be eliminated by tetracycline treatment, consistent with causation by a bacterial agent. Additionally, infection of a sterile Sxlf4 strain with Wolbachia from an outside source was sufficient to cause oogenesis and fertility. Wolbachia fails to suppress the closely related allele Sxlf5, showing that infection does not cause a general bypass of the requirement for Sxl in the female germ line, nor does infection generally suppress ovarian tumors. Remarkably, Sxlf18, whose molecular effects are very different from those of Sxlf4 and Sxlf5, is also suppressed, though to a lesser extent. Sxlf4 suppression mimics the natural situation observed in the wasp Asobara tapida, which requires Wolbachia infection for oogenesis (F. Dedeine et al. PNAS 98: 6247-6252). The fact that Wolbachia, a widespread manipulator of reproduction and sex determination in many species, interacts specifically with Sex-lethal, the master sex switch of the model organism D. melanogaster, should facilitate study of this fascinating category of host-parasite interactions, but also reveals an unanticipated complication in the genetic analysis of Sxl's germline functions.