Bombyx mori is a female-heterogametic organism (female, ZW; male, ZZ) that appears to have a putative feminizing gene (Fem) on the W chromosome. The paternally transmitted mutant W chromosome, Df(pSa + pW + od)Fem, derived from the translocation-carrying W chromosome (pSa + pW + od), is inert as femaleness determinant. Moreover, this Df(pSa + pW + od)Fem chromosome has been thought to have a female-killing factor because no female larvae having the Df(pSa + pW + od)Fem chromosome are produced. Initially, to investigate whether the Df(pSa + pW + od)Fem chromosome contains any region of the W chromosome or not, we analyzed the presence or absence of 12 W-specific RAPD markers. The Df(pSa + pW + od)Fem chromosome contained 3 of 12 W-specific RAPD markers. These results strongly indicate that the Df(pSa + pW + od)Fem chromosome contains the region of the W chromosome. Moreover, by using phenotypic and molecular markers, we confirmed that the Df(pSa + pW + od)Fem chromosome is connected with a partially deleted Z chromosome and that this fused chromosome behaves as a Z chromosome during male meiosis. Furthermore, we demonstrated that the ZZW-type triploid female having the Df(pSa + pW + od)Fem chromosome is viable. Therefore, we concluded that the Df(pSa + pW + od)Fem chromosome does not have a female-killing factor but that partial deletion of the Z chromosome causes the death of the ZW-type diploid female having the Df(pSa + pW ++ od)Fem chromosome. Additionally, our results of detailed genetic analyses strongly indicate that the female-killing chromosome composed of the Df(pSa + pW + od)Fem chromosome and deleted Z chromosome was generated by translocation between the Z chromosome and the translocation-carrying W chromosome, pSa + pW + od.