Abstract The polypeptide composition of highly purified polyoma virus was studied by SDS-polyacrylamide gel electrophoresis, and in agreement with the observations of Roblin et al. (1971), six structural polypeptides were found. An examination of the time course of synthesis of these polypeptides showed that the synthesis of VP4, 5 and 6, the small basic proteins, begins at about the same time as does the synthesis of viral DNA. The synthesis of VP1, the major viral polypeptide, was first detected 2 hr later, while that of the minor components VP2 and 3, whose appearance even in isolated nuclei, is obscured by a high background of cellular protein synthesis, could not be detected with confidence earlier than 5 to 6 hrs after the onset of DNA synthesis. Polypeptides VP4, 5, and 6 are not present in either naturally occurring or artificially produced empty capsids, but are structural components of pseudovirions. They have a relatively high lysine content, but contain no tryptophan, and in SDS-polyacrylamide gels were found to comigrate with certain of the histones isolated from uninfected mouse kidney cells. Histones, synthesized in mouse kidney cells prior to infection with polyoma virus, were shown to be incorporated into both progeny virions and pseudovirions.