Although serologic evidence for Hantaan (HTN) or Seoul (SEO) virus infection can be found in the vast majority of the more than 300 cases of hemorrhagic fever with renal syndrome (HFRS) oc- curring annually in Korea, approximately 4% of Korean patients with HFRS show a more than 4- fold higher antibody titer to Puumala (PUU) virus than to HTN or SEO virus by double-sandwich IgM ELISA. We now report the existence of a previously unrecognized arvicolid rodent-borne han- tavirus, designated Muju (MUJ) virus, in Eothenomys regulus (forrnerly called Clethrionomys regulus). Alignment and comparison of a 241 - nucleotide region of the G2 glycoprotein-encoding M segment and entire region of the nucleocapsid (N) protein-encodings S segment, amplified and sequenced from lung tissues of Eothenomys voles captured during October and November 1996 in Muju-kun. Cheonlabuk-do, Korea, indicated 79.5-83.4% and 71.2-76.7% sequence similarity, respectively, with PUU virus strains. At the amino acid level, MUJ virus diverged from PUU virus by 8.7 11.2% (G2) and 5.4-6.7% (N). Phylogenetic analysis indicated that MUJ virus was related to, but distinct from, other Old World arvicolid rodent-borne hantaviruses, including Khabarovsk and Tula viruses. Studies to determine the epizootiology and pathogenic potential of MUJ virus are in progress.