Three major species of viral DNA have been observed in cells infected by retroviruses: a linear, double-stranded copy of a subunit of viral RNA; closed circular DNA; and proviral DNA inserted covalently into the genome of the host cell. We have studied the structures of the unintegrated forms of avian sarcoma virus (ASA) DNA using agarose gel electrophoresis in conjunction with restriction endonucleases and molecular hybridization techniques. The linear duplex DNA is approximately the same length as a subunit of viral RNA (approximately 10 kb) and it bears natural repeats of approximately 300 nucleotides at its termini. The repeats are composed of sequences derived from both the 3' and 5' termini of viral RNA in a manner suggesting that the viral DNA polymerase is transferred twice between templates. Thus the first end begins with a sequence from the 5' terminus of viral RNA and is permuted by about 100 nucleotides with respect to the 3' terminus of viral RNA; the linear DNA terminates with a sequence of about 200 nucleotides derived from the 3' end of viral RNA. We represent this structure, synthesized from right to left, as 3'5'-----3'5'. Two closed circular species of approximately monomeric size have been identified. The less abundant species contain all the sequences identified in linear DNA, including two copies in tandem of the 300 nucleotide 3'5' repeat. The major species lacks about 300 base pairs (bp) mapped to the region of the repeated sequence; thus it presumably contains only a single copy of that sequence. The strategies used to determine these structures involved the assignment of over 20 cleavage sites for restriction endonucleases on the physical maps of ASV DNA. Several strains of ASV were compared with respect to these sites, and the sites have been located in relation to deletions frequently observed in the env and src genes of ASV.