An endogenous type C virus recently isolated from the Columbian black-tailed deer (Odocoileus hemionus) was used as a molecular probe to study the distribution of virus-related nucleotide sequences in cellular DNAs of mammalian species. By DNA-DNA hybridization, the most extensive homology was demonstrated between the viral complementary DNA and cellular DNA isolated from Odocoileus species. DNAs of representatives of other genera within the same family, Cervidae, were partially related to the virus, consistent with the phylogenetic relationship of these species to Odocoileus. O. hemionus viral sequences were also detected within cellular DNAs of members of a more distantly related artiodactyl family, Bovidae. These findings suggest the genetic transmission of type C viral genes within cervids and bovids for a period of at least 25 to 30 million years. There was no detectable nucleotide sequence homology between O. hemionus virus and representatives of other major groups of mammalian type C viruses. These results indicate that despite the known antigenic relatedness of mammalian type C viruses, the O. hemionus virus has diverged sufficiently to be considered the prototype of a separate group. By radioimmunological techniques, it was possible to detect and partially purify, from normal tissues of cervid species, antigens related to the major structural protein of the O. hemionus virus. The present findings, that O. hemionus virus has been genetically transmitted for millions of years and yet has maintained the ability to be expressed as infectious virus, argue for positive evolutionary selective pressures for the maintenance of type C viral genes.