The vertebrate egg envelope is composed of a family of related proteins, the zona pellucida (ZP) proteins, which are characterized by the presence of a conserved zona pellucida domain. Analysis of teleost fish ZP gene sequences has demonstrated that there are no direct orthologues of the mammalian ZPB and ZP1 genes, but that teleost fish contain multiple copies of two classes of genes (ZPXa and ZPXb) that are equally related to ZPB and ZP1. The two classes of genes are further distinguished by expression in liver or ovary, respectively, indicating there was probably an initial duplication event, followed by a switch to hepatic expression of one of the paralogues. This switch was followed in some species by additional amplification of one of the paralogues with the subsequent loss of the other. It is proposed that the expansion of the number of ZPXa and ZPXb genes and the acquisition of dual sites of synthesis are the result of an ancient polyploidization event, followed by additional species-specific gene amplifications.