The polypeptides of polyoma and SV40 virions are phosphorylated. An estimate of the amount of phosphorylation of the major virus capsid protein (VPI) has been made using two-dimensional gel electrophoresis to resolve phosphorylated from non-phosphorylated forms. The results suggest that in both polyoma and SV40 virions about 12% of VPI molecules are phosphorylated. In unassembled VPI molecules immunoprecipitated from extracts of infected cells the proportion is greater, about 33%. The possibility that phosphorylated VPI may form the penton proteins of the virus capsid is discussed.