Choriogenesis is the last step of insect oogenesis, a process by which the chorion polypeptides are produced by the follicular cells and deposited on the surface of oocytes in order to provide a highly specialized protective barrier to the embryo.  The essential features of chorion genes have yet to be clearly understood in the diamondback moth, Plutella xylostella, a worldwide Lepidoptera pest attacking cruciferous crops and wild plants.  In this study, complete sequences for 15 putative chorion genes were identified, and grouped into A and B classes.  Phylogenetic analysis revealed that both classes were highly conserved and within each, branches are also species-specific.  Chorion genes from each class were located in pairs on scaffolds of the P. xylostella genome, some of which shared the common promoter regulatory region.  All chorion genes were highly specifically expressed in the P. xylostella adult females, mostly in the ovary with full yolk, which is a crucial period to build the shells of the eggs.  RNAi-based knockdown of chorion-1, which is located on the Px_scaffold 6 alone, although had no effect on yolk deposition, resulted in smaller eggs and sharply reduced hatchability.  Additionally, inhibition of PxCho-1 expression caused a less dense arrangement of the columnar layers, reduced exochorion roughness and shorter microvilli.  Our study provides the foundation for exploring molecular mechanisms of female reproduction in P. xylostella, and for making use of chorion genes as the potential genetic-based molecular target to better control this economically important pest.