A 39-base oligonucleotide "guessmer" probe, based on partial N-terminal sequence analysis of the aspartate aminotransferase purified from Thermus aquaticus strain YT1, was used to screen a genomic library prepared from T. aquaticus DNA. A 1842 bp DNA fragment was isolated that proved to contain the coding sequence for the aspartate aminotransferase. The gene is 1152 bases long and codes for a protein of 383 amino acid residues. The amino acid sequence obtained showed 88.7%, 45.1% and 32.9% identity of sequence with those of thermostable aspartate aminotransferases from T. thermophilus, Bacillus YM2, and Sulfolobus solfataricus, respectively. It showed 39.1% identity with one of the gene products tentatively identified as aspartate aminotransferase from the methanogenic archaebacterium Methanococcus jannaschii. Neither the amino acid compositions nor the aligned amino acid sequences provides any obvious clue as to the origin of thermal stability in this group of enzymes.