The polypeptides of cells infected with a series of plaque isolates of the simian rotavirus SA11 were analyzed by SDS-PAGE. Altered electrophoretic migration of the major outer capsid glycoprotein (VP7) was found with independent virus stocks exhibiting gene products of VP7 ranging in apparent molecular weight from 35.5K to 38K. Similar differences in electrophoretic migration in the polypeptide precursor of the glycoprotein (pVP7) suggested that the heterogeneity resulted from mutations in the gene encoding the glycoprotein. The glycoprotein phenotype was stable on passage; the phenotypes were unchanged for 10 passages at high and low multiplicity. The biologic consequences of heterogeneity in the polypeptide are discussed.