Rabbit kidney vacuolating virus is similar to polyoma virus in the size and structure of the virus particle. The DNA of the virus is also similar to the DNA of polyoma virus, being supercoiled and circular with a molecular weight of approximately 3 million. The base composition of the DNA, 43% guanine plus cytosine (GC), is intermediate between that of polyoma virus DNA, 48% GC, and that of simian virus 40 DNA, 41% GC. It is concluded that rabbit kidney vacuolating virus should be placed with polyoma virus and simian virus 40 as a member of the polyoma group of viruses.