One to two year old deer in a herd of fallow deer began to show signs of weight loss, patchy alopecia, and occasional intermittent diarrhea. Several severely affected deer from the above age group were killed and samples were submitted for diagnostic testing. Paratuberculosis was confirmed in the index cases. In the remaining herd (70 deer), of 52 deer sampled, 51 were determined to be infected with M. paratuberculosis by at least one of the following diagnostic methods; fecal culture, tissue culture, and/or histopathology. As a diagnostic test, fecal culture detected 32 of 51 infected animals with an apparent sensitivity of 62%. Sensitivity of the fecal culture varied from a low of 45% in the most mildly infected deer to 91% in the most severely affected animals. The ileocecal junction was positive in 82% (range 71 to 100%) of cases depending on the age of the animal and severity of infection. In this herd, the palatine tonsil was positive by culture in 88% of 17 deer tested. Cultures were confirmed with an IS900 probe. Paratuberculosis infection and fecal shedding of organism were found in deer as young as 6 months of age. Culture confirmed that the 1 to 2 year old deer were the most severely infected. In this group, M. paratuberculosis occasionally caused disseminated infection. Affected tissues included the intestine, liver, multiple lymph nodes, and occasionally lung. Paddocks, on the affected farm, were divided into equal quadrants and the environment was sampled to determine the level of environmental contamination. A total of forty quadrants were sampled including the pond within the enclosure. Of the pasture quadrants sampled, 25 of 36 (69%) were positive for M. paratuberculosis. The inflow sediment from the pond was also positive.