The North American Red-backed vole, Clethrionomys gapperi, and the Eurasian Bank vole, C. glareolus, are probably derived from a common ancestor. They have been isolated from each other for a minimum of 20,000 generations, probably many more. Interbreeding experiments in the laboratory show that in that time only partial reproductive isolation has developed. They interbreed as frequently as do pure strain pairs, and produce litters of approximately the same size. Hybrids exhibit reduced viability, but this may be partly an artifact produced by the laboratory rearing conditions. If they ever made contact in nature the most likely outcome appears to be selection against hybrids leading to full speciation and the establishment of contiguous allopatry. It is recommended that they be considered semispecies, in the terminology of Mayr (1963), and retain their separate names.