Abstract: 【Aim】 Wolbachia bacteria are maternallyinherited endosymbionts frequently found in insects. They can alter their hosts’ reproduction via several forms. Cytoplasmic incompatibility (CI) is the most commonly reported phenotype caused by Wolbachia infection and induces developmental arrest of early embryos resulting from mating between Wolbachia-infected males and uninfected females. The molecular mechanisms of CI, however, are not clear currently. Our previous studies showed that the Mst84Db gene exhibited significant down-regulation in the 3rd instar larval testes of Drosophila melanogaster when infected by Wolbachia. The objective of this research is to further assay whether Mst84Dbgene is associated with CI. 【Methods】 In this study, we first synthesized theMst84Db dsRNA in vitro and microinjected into male flies. Then we tested the fertility of these injected males by crossing with wild type females. Gene expression levels were examined by quantitative RT-PCR (qRT-PCR). The embryonic phenotypes were analyzed by DAPI staining. 【Results】 The expression level of Mst84Db was significantly down-regulated since 24 h after injecting Mst84Db dsRNA. At 72 h after injection, the expression level of Mst84Db exhibited the maximum down-regulation relative to the control. The knockdown of Mst84Db in males significantly decreased the hatch rate of embryos when crossed with wild type females. This is similar to the phenotype in CI crosses. In addition, the embryonic cuticles showed almost no segmental development, indicating that some embryos stop development at early stages. Obvious CI phenotypes with asynchronized nuclear division and chromatin bridges were also observed in part of early embryos derived from Mst84Db down-regulated males. 【Conclusion】 Wolbachia infection may inhibit the expression of Mst84Db during spermatogenesis, thus reduces sperm fertility and leads to embryonic lethality. Down-regulation of Mst84Db in male Drosophila might be causally linked to CI.