Macrorhabdus ornithogaster is a newly described anamorphic ascomycetous yeast that has been reported to cause a chronic, debilitating disease in many species of birds, including poultry. Study of this organism is complicated by the limited ability to grow M. ornithogaster in vitro. In this study, we showed that the chicken can be used to amplify this organism and as a model to study its pathogenicity. An infection rate of 100% was achieved in day-old chicks orally inoculated with 10⁵ M. ornithogaster derived from the budgerigar (Melopsittacus undulatus). The organism was also determined to increase in number by greater than 10-fold 14 days after oral inoculation in these chicks. Chickens infected with M. ornithogaster demonstrated no sign of illness but had decreased feed conversion efficiency and consistent and characteristic histopathologic lesions in the proventriculus and isthmus of the stomach, suggesting that M. ornithogaster may represent a potential threat to the poultry industry.