The three-dimensional structure of bovine papillomavirus has been determined to 9 A resolution by reconstruction of high resolution, low dose cryo-electron micrographs of quench-frozen virions. Although hexavalent and pentavalent capsomeres form star-shaped pentamers of the major capsid protein L1, they have distinct high-resolution structures. Most prominently, a 25 A hole in the centre of hexavalent capsomeres is occluded in the pentavalent capsomeres. This raises the possibility that the L2 minor capsid protein is located in the centre of the pentavalent capsomeres. Inter-capsomere connections approximately 10 A in diameter were clearly resolved. These link adjacent capsomeres and are reminiscent of the helical connections that stabilize polyomavirus.