During the last 20 years deer farming has developed into a major pastoral industry in New Zealand. The national deer herd consists of approximately 1,000,000 animals. Johne's disease in farmed deer is rare and there have been only 16 bacteriologically confirmed cases in the last 5 years. This is in marked contrast to the high level of infection in cattle and sheep in New Zealand. Half of the 16 cases were identified at routine meat inspection of clinically normal deer. Gross lesions of Johne's disease are often caseous and similar to those caused by other members of the Mycobacterium avium complex or Mycobacterium bovis. Characterization of cervine strains of Mycobacterium paratuberculosis by restriction endonuclease analysis and DNA hybridization revealed two types, one identical to those found in New Zealand sheep and the other to those from cattle. Clinical Johne's disease in deer occurs very rarely but has often been observed in animals as young as 12 months of age. It appears that rare individuals are very susceptible to mycobacterial infections, including Johne's disease, while the majority of deer have a significant degree of resistance to infection with M. paratuberculosis.