Full-length cDNAs were sequenced for genome segment 6 from representative isolates of all 24 bluetongue virus (BTV) serotypes. Segment 6 is 1635 to 1645 nucleotides in length with a single open reading frame in all isolates. The deduced VP5 protein sequence is 526 amino acids long for most BTVs, except BTV-15 (isolate number RSArrrr/15), which is 527 amino acids long, and BTV-12 (isolate number RSArrrr/12) and BTV-22 (isolate number Nig1982/11), which were 529 amino acids long. Sequence comparisons have revealed the level of genetic diversity in segment 6, between different BTV serotypes and between isolates within a single serotype. The resulting sequences can be used to design RT-PCR primers for amplification and identification of segment 6 from new BTV isolates, providing potentially valuable diagnostic and research tools. These data are available on the international databases and accession numbers are listed on the website of the Institute for Animal Health (iah.bbsrc.ac.uk/dsRNA_virus_proteins/ btv_sequences.htm).