A procedure has been developed to map the genetic elements of avian tumor virus RNA, which has a molecular weight of about 3 X 10(6) daltons and a poly(A) sequence at the 3' end. For this purpose, about 30 RNase T1-resistant oligonucleotides were ordered relative to the 3'-poly(A) terminus of the RNA, to construct an oligonucleotide map of viral RNAs. A cluster of seven envelope gene (env)-specific oligonucleotides, identified by their absence from the otherwise very similar oligonucleotide map of an envelope-defective deletion mutant (which lacks the major viral glycoprotein), mapped at a distance of 0.9 to 1.6 X 10(6) daltons from the poly(A) end of sarcoma virus RNA. A cluster of three sarcoma gene (src)-specific oligonucleotides, identified by their absence from the otherwise nearly identical oligonucleotide map of a transformation-defective deletion mutant mapped at a distance of 0.2 to 0.6 X 10(6) daltons from the poly(A) end of sarcoma virus RNA. The oligonucleotide maps of sarcoma viruses and of related deletion mutants were the same from the poly(A) end up to 0.2 X 10(6) daltons and included one terminal oligonucleotide, termed C, which is found in all avian tumor viruses tested so far. Preliminary mapping experiments ordering the src-specific and env-specific oligonucleotides of recombinants, selected for sarcoma and envelope genes of different parents, agree with those obtained by comparing maps of wild type viruses and deletion mutants. A partial genetic map consistent with these results suggests that the src gene maps between the env gene and the 3'-poly(A) end of viral RNA. This map reads: poly(A)-src-env-(pol, gag).