The complete genome of the hamster papovavirus (HaPV) which was isolated from virions found in multiple skin tumors of the Syrian hamsters was measured by electron microscopy and cloned in Escherichia coli using the certified plasmid vector pBR322. The cloned viral DNA were characterized by digestion of the recombinant DNA with various restriction enzymes followed by comparison of their electrophoretic mobilities in agarose gels with that of similarly digested uncloned DNA and by electron microscopy to determine the genome size of cloned HaPV DNA. The restriction enzyme analysis of the cloned HaPV DNA showed the same cleavage pattern as the corresponding fragments from the uncloned DNA. No major insertions or deletions could be detected by heteroduplex analysis between cloned HaPV DNA and the starting material. The estimated genome size of 5.52 kb for HaPV DNA is approx. 300 bases larger than those determined for other known papovaviruses as SV40 or polyoma.