Infection of the Bursa of Fabricius with Cryptosporidium baileyi was diagnosed in a group of hand reared Red-breasted Merganser (Mergus serrator) ducklings during one breeding season in a German zoological garden.The birds had died in an emaciated and anaemic state after problems with spontaneous feeding. The bursae were infected with moderate to high numbers of cryptosporidia, which were associated with hyperplasia, degeneration and sloughing of the affected epithelial cells and mild heterophilic bursitis, lesions typically seen in bursal cryptosporidiosis in other avian species. In addition, lymphatic tissue was nearly absent, which was probably caused by chronic stress and malnutrition related to the rearing of these highly stress-sensitive birds in an artificial environment. Companion ducklings from stress-resistant, spontaneously feeding species from the zoological collection were used to calm the Mergansers, but may have introduced the cryptosporidia into the rearing boxes. Another possible source for the introduction of C. baileyi were adult mallards (Anas platyrhynchos), which were used to increase the hatching rate by a phase of natural breeding in the middle third of the incubation period of the Merganser eggs.