The gene encoding the thermostable phenylalanine dehydrogenase [EC 1.4.1.-] of a thermophile, Thermoactinomyces intermedius, was cloned and its complete DNA sequence was determined. The phenylalanine dehydrogenase gene (pdh) consists of 1,098 nucleotides and encodes 366 amino acid residues corresponding to the subunit (Mr 41,000) of the hexameric enzyme. The amino acid sequence deduced from the nucleotide sequence of the pdh gene of T. intermedius was 56.0 and 42.1% homologous to those of the phenylalanine dehydrogenases of Bacillus sphaericus and Sporosarcina ureae, respectively. It shows 47.5% homology to that of the thermostable leucine dehydrogenase from B. stearothermophilus. The pdh gene was highly expressed in E. coli JM109, the amount of phenylalanine dehydrogenase produced amounting up to about 8.3% of that of the total soluble protein. We purified the enzyme to homogeneity from transformant cells in a day, with a 58% recovery.