Polar bear (Ursus maritimus) morbillivirus infection was initially reported by Follmann and co-workers in 1996, based upon serologic results using canine distemper virus (CDV). The impetus for the evaluation of polar bear populations for morbillivirus infections was prompted by epidemics of canine distemper-like disease in seal populations in the north Atlantic regions of Greenland, Europe, and Russia. Since marine morbilliviruses have been further characterized into three major species, phocine distemper virus (PDV), dolphin morbillivirus (DMV) and porpoise morbillivirus (PMV), it was of value to determine the origin of the polar bear infection. One hundred serum samples were selected from a group of sera collected from regions of Alaska and Russia and tested by differential serum neutralization assay against the three major marine morbilliviruses and CDV, to determine the predominant virus infecting the polar bear. Polar bears had higher serum antibody titers to CDV than they did to PDV, DMV, and PMV. These data suggest that polar bears are being infected with a morbillivirus of terrestrial origin. Furthermore, based on the high serum antibody prevalence in the population, the virus may be indigenous to the polar bear and not necessarily the result of interspecies transmission from other arctic mammals susceptible to CDV and/or marine morbilliviruses.