The genome of an avian oncornavirus was altered after infection of a heterologous host. This was studied with avian sarcoma virus B-77 in duck embryonic fibroblasts (DEF) and chicken embryonic fibroblasts (CEF). To detect alteration of the viral genome, we hybridized 35S B-77 RNA with normal duck DNA by either one of two techniques:when viral RNA was in excess and when DNA was in excess. The RNA of B-77 passaged only in gs minus chf minus CEF does not have homology with duck DNA. However, after several passages of B-77 through DEF the viral genome acquired duck specific RNA sequences. After 4 and 10 passages, B-77 RNA acquired 2.2 and 6.6%, respectively, complementarity to normal duck DNA. The duck specific RNA sequences were found to be covalently linked to the B-77 RNA genome. Also, the host specific sequences acquired by the virus appear to be from a region of the duck DNA which is repeated four to six times per cell. After 5 back passages in CEF some of the duck specific RNA sequences in the viral genome were lost.