Abstract Bovine leukemia virus (BLV) proviruses, harbored by the productively infected fetal lamb kidney (FLK-BLV) cell line, were cloned in bacteriophage λL47 The nucleotide sequence of the proviral long terminal repeats (LTR) with flanking cell and virus DNA have been determined. The BLV LTR is 531 by in length and is bounded by the dinucleotides 5′-TG… CA-3′, which are part of a 3-bp inverted repeat. The integrated provirus is flanked by 6-bp direct repeats of cellular DNA. A tRNA pro primer binding site is present starting 2 by downstream of the 5′ LTR. In addition to sequencing integrated proviral DNA clones, the nucleotide sequence of a cDNA clone, representing the the' end of genomic viral RNA, was determined; thus revealing the RNA polyadenylation site and R:U5 boundary within the LTR. Unlike most other retroviruses, a consensus polyadenylation signal, “AATAAA,” is not located proximal to the BLV polyadenylation site. The RNA initiation site, defining the U3:R boundary, was located in the BLV LTR by S1 nuclease mapping. This site is approximately 25 by downstream of an A + T-rich region which probably encompasses a Goldberg-Hogness (“TATAA”) box and about 90 bp downstream of a potential “CCAAT” box. The BLV LTR possesses a U3 region of 204 bp, an unusually long R region of 241 bp, and a U5 region of 86 bp.