A genomic region of 41,045 bp encompassing the 3'-end of the sheep T cell receptor beta chain was sequenced. Extensive molecular analysis has revealed that this region retains a unique structural feature for the presence of a third D-J-C cluster, never detected in any other mammalian species examined so far. A total of 3 TRBD, 18 TRBJ and 3 substantially identical TRBC genes were identified in about 28kb. At 13kb, downstream from the last TRBC gene, in an inverted transcriptional orientation, lies a TRBV gene. Sequence comparison and phylogenetic analyses have demonstrated that the extra D-J-C cluster originated from an unequal crossing over between the two ancestral TRBC genes. Interspersed repeats spanning 22.2% of the sequence, contribute to the wider size of the sheep TRB locus with respect to the other mammalian counterparts, without modifying the general genomic architecture. The nucleotide and predicted amino acid sequences from peripheral T cells cDNA clones indicated that the genes from cluster 3 are fully implicated in the beta chain recombination machinery. Closer inspections of the transcripts have also shown that inter-cluster rearrangements and splice variants, involving the additional cluster, increase the functional diversity of the sheep beta chain repertoire.