A wild muskrat (Ondatra zibethicus) found moribund in Illinois (USA) had minimal meningitis and pleuritis, probably of bacterial origin. There were large, basophilic, intranuclear inclusion bodies within scattered enterocytes. The inclusions were microscopically typical of those produced by adenoviruses, and ultrastructurally were intranuclear paracrystalline arrays of virus particles with characteristics of adenoviruses. The significance of the adenovirus infection in this muskrat is unknown.