Infection by a herpesvirus producing cytomegalic disease in the prostate was demonstrated in the dasyurid marsupials, Phascogale tapoatafa and Antechinus stuartii. The prevalence of lesions among the latter was highest in mature animals during breeding, when the animals are known to be under stress, and in animals treated daily with high levels of exogenous corticosteroid. Occasional cytomegaly was observed in the kidneys of some A. stuartii and may represent a site of latent infection. Virus particles of herpes type were demonstrated by electron microscopy in the nucleus and cytoplasm of infected prostatic cells. It is suggested that the infection may be venereal.