Although Mycobacterium avium subsp. paratuberculosis infection has had its greatest effect on domestic agricultural animal species, it can also have a significant impact on wildlife species. More cases of infection are being reported, and because of its ability to elude immunologic control and to persist in the environment, M. paratuberculosis may spread within and among captive and free-ranging wildlife populations in the absence of organized control programs. Studies to improve our ability to detect the organism in biologic samples such as milk, blood, and manure through immunomagnetic separation, automated culture methods, and improved polymerase chain reaction procedures are underway in several countries. Studies of the organism's genetic components, virulence factors, and antigens support the development of new diagnostic tools and vaccines.