Cytomegalovirus (CMV) has been considered a difficult virus to isolate and study because of its apparent lability. Investigations of the thermostability of strains have yielded discrepant results, some of which appear to be due to the diluents used. The stability of three CMV strains at 37°, 25° and 3°C has been studied here. Virus was diluted into either distilled water or a transport medium containing trypticase soy broth and bovine albumin. The three strains behaved in a similar manner. Virus was inactivated at the rate of one log10 per day at 37°C, but at 25° and 3°C the original infectivity was maintained for 2 and 7–10 days respectively. Virus was more stable at 37°C in distilled water than in transport medium.