D-Glyceraldehyde-3-P:NAD oxidoreductase (phosphorylating) (E.C. 1.2.1.12) (abbr. GPDH) is a ubiquitous enzyme, found in most living systems, where it functions in the glycolytic pathway. There were several reasons for studying the enzyme in thermophilic bacteria. First, at the time this enzyme was characterized in the obligate thermophile Bacillus stearothermophilus (1503), no intracellular enzymes from thermophilic bacteria had been isolated in highly purified form. Second, the enzyme had been well characterized in many other systems, thereby providing a sound basis for comparison with the thermophilic enzyme. Third, GPDH is almost always present within cells at very high concentrations, thereby providing a large quantity of potential material necessary for extensive physicochemical analyses. Finally, was the demonstration by Amelunxen and Lins (1), that the enzyme in crude extracts from B. stearothermophilus was indeed remarkably thermostable, as seen in Table I.