Channel catfish, Ictalurus punctatus, T cell receptors (TCR) gamma and delta were identified by mining of expressed sequence tag databases, and full-length sequences were obtained by 5'-RACE and RT-PCR protocols. cDNAs for each of these TCR chains encode typical variable (V), diversity (D), joining (J), and constant (C) regions. Three TCR gamma V families, seven TCR gamma J sequences, and three TCR gamma C sequences were identified from sequencing of cDNA. Primer walking on bacterial artificial chromosomes (BACs) confirmed that the TRG locus contained seven TRGJ segments and indicated that the locus consists of (V gamma 3-J gamma 6-C gamma 2)-(V gamma 1(n)-J gamma 7-C gamma 3)-(V gamma 2-J gamma 5-J gamma 4-J gamma 3-J gamma 2-J gamma 1-C gamma 1). In comparison for TCR delta, two V families, four TCR delta D sequences, one TCR delta J sequence, and one TCR delta C sequence were identified by cDNA sequencing. Importantly, the finding that some catfish TCR delta cDNAs contain TCR V alpha-D-J delta rearrangements and some TCR alpha cDNAs contain V delta-J alpha rearrangements strongly implies that the catfish TRA and TRD loci are linked. Finally, primer walking on BACs and Southern blotting suggest that catfish have four TRDD gene segments and a single TRDJ and TRDC gene. As in most vertebrates, all three reading frames of each of the catfish TRDD segments can be used in functional rearrangements, and more than one TRDD segment can be used in a single rearrangement. As expected, catfish TCR delta CDR3 regions are longer and more diverse than TCR gamma CDR3 regions, and as a group they utilize more nucleotide additions and contain more nucleotide deletions than catfish TCR gamma rearrangements.