In trematodes, vitelline precursor proteins are required for eggshell formation. A cDNA clone of Clonorchis sinensis (CsVpB1) was selected from an EST pool, encoding a polypeptide of 245 amino acids. The CsVpB1 polypeptide demonstrated homology with vitelline precursor proteins from trematodes with high sequential identities. In a phylogenic tree, CsVpB1 clustered with trematode VpB proteins. The CsVpB1 polypeptide was found to be rich in tyrosine residues, including putative pre-dihydroxyphenyl alanine (DOPA) residues, involved in cross-linking of the precursor proteins. Mouse immune sera were raised against a recombinant CsVpB1 protein. In adult C. sinensis, CsVpB1 protein was exclusively localized in vitelline follicles. Based on these results, the CsVpB1 cDNA is believed to encode a VpB of C. sinensis.