DNA from a third isolate of the WW strain of BK virus was prepared from human urine and was cloned into the BamHI site of pBR322. Restriction mapping showed a number of changes relative to the prototype BK viral DNA, including the presence of a new PstI, an SstI, and an AvaII site and the loss of a HindIII, an SstI, and an NcoI site. There is evidence for more extensive changes in the vicinity of the origin of replication.