A cDNA encoding the Pacific oyster, Crassostrea gigas, estrogen receptor (cgER) was cloned using degenerate PCR primers. The open reading frame predicted 485 amino acid residues. Comparisons of the amino acid sequence of cgER with other mollusk ERs show high similarities of the C domain (95-97%), and the E domain (56-66%). The amino acid sequence of the C domain of cgER shows 86 and 89% identity with the respective sequences of human ER-alpha and ER-beta. The amino acid sequence of the E domain of cgER shows 45% identity with those of human ER-alpha and ER-beta. The phylogenetic analysis indicated that the cgER is an ortholog of the other mollusk ERs. In the C domain, the positions of cysteine residues and other residues around them that constitute the two zinc finger motifs and the P-box are conserved. The cgER mRNA was expressed in various tissues including the ovary. Reporter gene assay revealed that cgER is unresponsive to estrogen. This result is similar to those of other mollusk ERs. ER immunoreactivity was localized mainly in the nuclei of follicle cells, the site of vitellogenin synthesis, and in oocytes. This result suggests that cgER could work as a nuclear receptor.