The organization and sequences of the human beta-chain T-cell receptor diversity, joining, and constant region segments are described. The beta chain of the human T-cell receptor, analogous to the mouse counterpart, consists of two distinct constant region genes approximately equal to 10 kilobases apart. The two constant region genes, C beta 1 and C beta 2, are very similar not only in sequence but also in genomic organization. The coding sequences of each of these C beta constant region genes are divided into four exons. The first two exons encode most of the extracellular constant domain. The third exon encodes a major part of the presumed transmembrane portion, and the last exon contains the cytoplasmic coding sequence as well as 3' untranslated sequences. Except for a stretch of approximately equal to 95 highly conserved nucleotides extending 3' of the first exon of the C region genes, little homology can be found between the intron sequences of C beta 1 and C beta 2. A small cluster of joining region (J beta) gene segments is located approximately equal to 5 kilobases upstream of each of these two constant regions. The first cluster, J beta 1, contains six functional J gene segments while the second, J beta 2, contains seven functional J gene segments. In addition, diversity region (D beta) gene segments are located approximately equal to 600 base pairs upstream of each J beta. Recombinational signals containing highly conserved heptamer and nonamer sequences separated by 12 or 23 bases are found adjacent to all of these D beta and J beta gene segments. These signal sequences are thought to be involved in the somatic recombination processes. These results indicate that what appears to be a gene duplication event giving rise to these two distinct regions must have arisen a long time ago in the evolution of this gene locus.