A total of 41 Mycobacterium tuberculosis strains from patients of Vietnamese origin were analyzed by Southern blot hybridization with two different probes, IS6110 (Otal, I., et al., J. Clin. Microbiol. 29:1252-1254, 1991; Ross, B. C., et al., J. Clin. Microbiol. 30:942-946, 1992; Thierry, D., et al., J. Clin. Microbiol. 28:2668-2673, 1990; van Soolingen, D., et al., J. Clin. Microbiol. 29:2578-2586, 1991) and pTBN12 (Ross, B. C., et al., J. Clin. Microbiol. 30:942-946, 1992). The restriction fragment patterns of nine of these strains were virtually identical when the pTBN12 probe was used; five strains had a single copy of IS6110, and four strains failed to hybridize with the IS6110 probe. This relatively high frequency of strains with no or one copy of IS6110 suggests that the usefulness of IS6110 for epidemiological study may be limited in certain populations.