During a nearby construction project, a sudden decrease in food intake and guano production occurred in an outdoor colony of big brown bats (Eptesicus fuscus), and one animal was found dead. Investigation revealed that the project was generating a large amount of noise and vibration, which disturbed the bats' feeding. Consequently the bats were moved into an indoor enclosure away from the construction noises, and the colony resumed eating. Over the next 3 wk, additional animals presented with clinical signs of lethargy, weight loss, ecchymoses, and icterus and were necropsied. Gross necropsy of the affected bats revealed large, pale yellow to tan, friable livers with rounded edges that floated when placed in 10% neutral-buffered formalin. Some bats had ecchymoses on the webbing and skin and gross perirenal hemorrhage. Histologic examination showed hepatic and renal tubular lipidosis. The clinical and pathologic signs of hemorrhage and icterus were suggestive of hepatic failure. Hepatic lipidosis was attributed to stress and inappetence associated with environmental perturbations. Once the environmental stressor was removed, the colony morbidity and mortality decreased. However, 2 y later, a series of new environmental stressors triggered additional deaths associated with hepatic lipidosis. Over a 9-y period, 21 cases of hepatic lipidosis were diagnosed in this bat colony.