On the basis of its genome organization the hamster polyomavirus (HaPV) is closely related to the murine polyomavirus Py. But HaPV infection, in contrast to Py infection, gives rise to two different tumor types; depending on the hamster strain used for infection, HaPV induces either epitheliomas or lymphomas. Although the HaPV virions were shown to be similar to those of Py and SV40, more precise information about the structure and protein composition of the HaPV capsid was still missing. Here we describe the primary structure of the capsid protein-encoding HaPV genes and the structure and protein composition of the HaPV capsid. Virions isolated from epitheliomas in HaPV-infected hamsters were shown by electron microscopy to be spherical particles with the typical icosahedral structure of polyomaviruses. However, in contrast to the capsids of SV40 and Py, a T = 7 laevo symmetry of HaPV capsids was observed. Separation of HaPV virions in SDS polyacrylamide gels and Western blotting with VP1-specific antisera identified VP1 as the major capsid protein species corresponding in its molecular weight to the predicted value of 41.8 kDa. Because of the presence of two potential translational initiation sites in the VP1 gene, the N-terminal amino acid sequence of virion VP1 was determined and found to start at the second initiation site. The amino acid homologies of HaPV capsid proteins shared with Py varied between 65.5% (VP1), 45.4% (VP3) and 44.6% (VP2), whereas the homologies to the relevant proteins of other polyomaviruses were found to range between 49.6-57.9% for VP1 and 28.9-41% for VP2/VP3.