The genome of ring-necked pheasant virus, an avian oncovirus, is largely homologous to the genomes of chicken oncoviruses except for a specific nonhomology in env, the gene coding for the surface glycoprotein of the virion (J. Tal, D. J. Fujita, S. Kawai, H. E. Varmus, and J. M. Bishop, J. Virol. 21:497--505, 1977). We have used this nonhomology between ring-necked pheasant virus and chicken oncoviruses in electron microscopic studies of heteroduplex molecules. The env-specific region of nonhomology is 1.5 to 1.7 kilobases in length. Its 3' boundary is located 0.6 to 0.7 kilobases from the 3' end of the genome in transformation-defective viruses and 2.5 kilobases from the 3' end in nondefective avian sarcoma viruses. Comparison of several strains of avian oncoviruses shows that the 3' half of this env region is conserved, while the 5' half is more diverged. A small area at the very 3' end of env also shows divergence between different avian oncoviruses. We found no evidence for the presence of a previously unrecognized gene between env and src. An electrophoretic comparison of the glycoproteins from various avian oncoviruses shows that those of ring-necked pheasant virus and Chinese quail virus differ in molecular weight from the glycoproteins of the chicken oncoviruses.