cDNAs encoding two subtypes of the vitamin D receptor (VDR) are cloned from a teleost (flounder, Paralichthys olivaceus). This is the first report of VDR subtypes in fish. Flounder VDR (fVDR) a and b share 86% identity at the amino acid level. With human (h), rat, mouse, quail, and Xenopus VDRs, fVDRa shares 72%, 71%, 71%, 69%, and 71% identity, and fVDRb shares 70%, 69% 69% 67% and 68% identity, respectively. The peptide sequences of the DNA-binding domain (DBD and hormone-binding domain (HBD) of both subtypes have particularly high homology to those of the tetrapods; e.g. 92% identity for DBP and 74% for HBD between fVDRa and hVDR In an evolutionary tree constructed with peptide sequences of VDRs and related members of the nuclear receptor superfamily, fVDRa and b are more closely related to each other than to other molecules, and situated in the cluster of VDRs at a position which corresponds well with the evolutional position of fish in the vertebrates. Additional independent genome duplication which is thought to have occurred in ray-finned fish phylogeny may explain the existence of two subtypes of VDR in flounder.