Mitochondrial DNA samples from each of 21 humans of diverse racial and geographic origin were digested with each of 18 restriction endonucleases. The sizes of the resulting DNA fragments were compared after gel electrophoresis. No differences among the samples were detected in digest with 7 of the enzymes. Analysis of digests with the remaining enzymes showed one or more differences. Each of the 21 samples could be characterized individually on the basis of these digests. All between-sample differences could be explained by single base substitutions. No evidence for sequence rearrangements (inversions, transpositions) was obtained. Fourteen of the site alterations were shared by two or more samples; six of these were shared between races. The data indicate that individuals differ from a postulated ancestral mtDNA sequence at 0.18% of their base pairs. On the basis of an estimated rate for base substitution of 1% per 10(6) years [Brown, W. M., George, M., Jr. & Wilson, A. C. (1979) Proc. Natl. Acad. Sci. USA 76, 1967-1971], Homo sapiens could have speciated or passed through a severe population constriction as recently as 180,000 years ago. The data suggest that group-specific patterns of cleavage exist. The high resolution and precision afforded by this method of analysis makes possible the investigation of many questions concerning human population genetics, evolution, and recent history.