Avian sarcoma virus 1 (ASV-1) was isolated from a spontaneous sarcoma of an adult chicken. It is a replication-defective virus that induces fibrosarcomas in young chickens and oncogenic transformation in cultured chick embryo fibroblasts. The ASV-1 genome has been cloned in the lambda gt WES.lambda B vector from closed circular viral DNA extracted from infected cells. The nucleotide sequence of the oncogene insert in the ASV-1 genome has been determined. It is virtually identical to the sequence of the oncogene crk recently discovered in the avian sarcoma virus CT10. ASV-1 and CT10 are two independent retrovirus isolates carrying the crk oncogene.