An epizootic of fatal illness among infant common tree shrews (Tupaia glis) occurred in a research colony. Affected tree shrews were dehydrated, and ingested milk substitute remained in their stomachs for several hours without being digested. Twelve of 15 tree shrews born during a 3-month period died. The 4 tree shrews we were able to examine had intranuclear inclusions in the epithelium of the stomach and intestines, with mild to moderate multifocal necrosis. One tree shrew also had nasal epithelial inclusions and multiple foci of hepatocellular and biliary necrosis, with inclusions. The causative virus was identified as an adenovirus.