We have compared the sequences of the entire genomes of bovine leukemia virus (BLV) and human T-cell leukemia virus type I (HTLV-I). Both the gag and pol genes show overall strong homologies between the two retroviruses, indicating their close evolutionary relationship. However, a surface glycoprotein portion of the env gene shows little if any homology, probably reflecting a difference in their host range. These retroviruses appear to harbour a gag precursor-cleaving protease, a not yet experimentally identified viral protein, between their gag and pol genes. Most interestingly, the 3' end portion of the BLV genome (designated pXBL) contains a long open reading frame that has a typical protein-coding property. The product of this open reading frame has now been identified as a protein of 38,000 daltons, which is produced by a spliced mRNA. We note that its amino acid sequence shows appreciable homology, especially in its N-terminal quarter, to that of the HTLV-I counterpart (pX), and we thus suggest that BLV pXBL and HTLV-I pX has diverged from a common ancestral gene. Finally and very importantly, comparisons of the best conserved pol sequences and overall genomic organizations between BLV and several other oncoviruses allow us to propose that BLV and HTLV-I constitute a novel group of Oncovirinae, designated here as type "E."