We determined the nucleotide sequences of the coding region for the M gene in seven strains of measles virus (MV) that were isolated in Japan between 1984 and 1993. The mutation found among the seven differed from those of laboratory strains. Many of these mutations were the same as those that are characteristic of SSPE viruses. Thus, we suggest that the mutations that have been considered specific to SSPE virus are in fact consensus among prevailing MV.