To characterize the genetic properties of currently circulating strains of wild-type measles viruses, we constructed and sequenced cDNA clones of the nucleoprotein (N) and matrix (M) genes of wild-type strains isolated between 1958 and 1989. The N and M genes of wild-type isolates from the prevaccine era (before 1964) were highly related to each other and to the N and M genes of a currently used measles vaccine strain, Moraten. The N and M genes of these viruses differed by no more than 0.5% at the nucleotide level. In contrast, the N and M genes of wild-type viruses isolated between 1977 and 1989 showed genetic drift, with the greatest amount of drift occurring in the viruses isolated from recent cases in the United States. Overall, the M genes were slightly more conserved at the nucleotide level (2.6% nucleotide, 3.3% amino acid) than the N genes (4.8% nucleotide, 3.4% amino acid). Alignment of the predicted protein sequences of the N genes revealed two regions of amino acid heterogeneity. The evolutionary patterns for the N and M genes suggested that the wild-type viruses isolated in the United States in 1989 were more related to wild-type viruses isolated in the United Kingdom between 1983 and 1988 than to viruses isolated in the United States in 1983.