Cytomegaly and intranuclear inclusion bodies were observed in the renal collecting duct epithelium in three of four wild caught platypuses (Ornithorhynchus anatinus) from New South Wales using light and electron microscopy during routine pathological studies. Non-enveloped, spherical virions measuring about 80 nm in diameter were present in the nucleus and cytoplasm of affected cells as well as in the lumen of the renal tubule. A single enveloped virion measuring about 150 nm in diameter was found. There was no serological evidence of infection with cytomegalovirus (AD169 antigen) or adenovirus (mammalian and avian group antigens) in any of the platypuses. Although the identity of the virus was not confirmed, it was probably an adenovirus based on morphological grounds. The infection appeared to have little effect on the host.