Nucleic acid hybridization studies using clones retroviral DNA were used to examine the evolution and organization of primate and carnivore retroviruses. These data were compared to those obtained on the evolution of unique sequence cellular DNA among these two mammalian orders. DNA fragments related to the gag and pol regions of the endogenous RD-114 and FeLV retroviruses were found in just six closely related species of Felidae. Fl hybrids from crosses of virogene-positive and virogene-negative cats were bred back to the virogene-negative parent. The cellular DNA extracted from 42 of these backcross offspring were analyzed after restriction enzyme digestion with probes specific to the two feline virogenes. The results demonstrate that viral sequences from both retrovirus families are dispersed on multiple chromosomes throughout the cat genome. Genetic mapping of sequences from both families of viruses in the backcross animals will be accomplished by correlating the occurrence of unique viral restriction fragments with feline isozymes previously assigned to specific cat chromosomes. A phylogenetic tree of carnivore evolution has also been established by molecular hybridization techniques; it differs in many respects from the classical relationships derived by anatomical considerations and the fossil record. Four molecular approaches, including DNA hybridization, isozyme genetic distance, immunological distance and G-banded karyology, were applied to determine the taxonomic status of the giant and lesser panda. A consensus phylogeny is derived that reveals that the lesser panda's ancestors emerged at the time of the procyonid-ursid divergence and that the giant panda diverged from the ursid lineage of the carnivores contemporaneously with the human-gibbon divergence among the primates.