A cDNA encoding the masu salmon, Oncorhynchus masou, estrogen receptor $alpha$ (msER$alpha$) was cloned from the pituitary gland by polymerase chain reaction (PCR). This cDNA contains an open reading frame encoding 513 amino acid residues, and the calculated molecular weight of this protein is about 56,430 Dalton. The amino acid sequences of the DNA binding and ligand binding domains of msER$alpha$ showed high homology to those of other fish species (84-100%). Reverse transcription PCR analysis showed that the mRNA level of msER$alpha$ in the pituitary was slightly higher in estradiol-17$eta$(E2) injected masu salmon than that of control fish. To test the biological activity of msER$alpha$, the cDNA was ligated to a mammalian expression vector and transfected into a gonadotrope-derived cell line, L$eta$T2, with a reporter plasmid including estrogen responsive element. Expression of the reporter protein, luciferase, was E2 and msER$alpha$-dependent. The masu salmon ER$alpha$ is structurally conserved among teleost species and functions as a transcriptional activator in the pituitary cells.