Serum antibody prevalence of parainfluenza 3 virus in the free-ranging Delta bison (Bison bison) herd which is found near Delta Junction, Alaska (USA), increased from 0% to 100% during the period 1977 to 1984. Domestic cattle are hypothesized as the source for the infection. There has been no clinical disease or decrease in productivity in this bison herd since establishment of the infection.