To study the epizootiology of malignant catarrhal fever viruses (MCFV), serum and spleen samples collected in 2004-2011 from a free-ranging musk ox (Ovibos moschatus) population in Dovrefjell, Norway, were examined. Sera were tested for antibodies against MCFV by competitive enzyme-linked immunosorbent assay and spleen samples were examined by a consensus polymerase chain reaction (PCR) for herpesviral DNA and sequencing identification. The study included 101 musk oxen, of which 61 were examined with both tests. Antibodies against MCFV were found in 65 of 72 musk oxen (90%). Antibody prevalence increased with age from 67% in calves to 96% in adults. We detected MCFV DNA by consensus PCR in 67 of 90 spleen samples tested (74%). The prevalence of PCR-positive musk oxen increased with age from 60% in calves to 81% in adults. Fifty (82%) of the 61 animals subjected to both PCR and serology were positive in both tests. Sequencing analysis showed that all PCR-positive animals were infected with a MCFV previously identified in musk oxen in North America. The results suggest that MCFV-Muskox is enzootic in the Dovrefjell musk ox population and that many calves seem to be infected early in life. This is the first report of MCFV-Muskox infection in free-ranging musk ox outside North America.