DNA, isolated from more than 100 mycobactin-dependent strains of Mycobacterium paratuberculosis, was digested with restriction endonuclease Pst I and hybridized with the fragment IS900. The strains were isolated from more than 80 domestic ruminants (cattle, sheep and goats) in 30 herds and 9 countries, and from patients suffering from Crohn's disease (strain Linda, U.S.A., and Fryba, Czech Republic). Three types of strains, tentatively designated A (bovine, ovine and Linda), B (bovine, ovine and Fryba) and C (ovine and caprine) were identified by RFLP. Differences in RFLP patterns were found between the strains isolated from mesenteric lymph nodes and from intestinal mucosa of one cow and one sheep. A single type was identified in cattle and sheep kept in one farm. All the three RFLP types were found in a mixed sheep-and-goat farm in the Czech Republic with frequent purchases and sales of animals. A within-region dominance of a single type was observed in strains isolated in the Czech and the Slovak Republics. The strain Fryba belonged to the RFLP type identified in strains isolated in the same region from more than 10 herds of cattle and sheep. Type B strains were isolated in 1993 from a herd imported in 1990 and kept in a strict separation, while only the type A was identified in the remaining cattle and sheep herds in the same region during the past 6 years.