The DNA of the BK virus (BKV) human papovavirus was found to be heterogeneous, consisting of at least four discrete species of DNA. Only the largest of these four species, BKV DNA (i), which has a molecular weight calculated to be 96% that of simian virus 40 (SV40) DNA, was infectious. Homogeneous preparations of BKV DNA were obtained, however, from virions purified after low multiplicity infections of human embryonic kidney cells. BKV DNA (i) was shown to contain a single R-Eco RI and four R-Hind cleavage sites. The R-Eco RI site was localized in the largest R-Hind cleavage fragment. Radiolabeled BKV DNA reassociated slightly faster than SV40 DNA; 20 to 30% polynucleotide sequence homology was demonstrated between the genomes of BKV and SV40 when the reaction was monitored by chromatography on hydroxyapatite.