The allosteric transhydrogenase from P. aeruginosa (EC 1.6.1.1) acts as an essential link between carbohydrate catabolism and respiratory chain (Widmer and Kaplan, Biochemistry 15, 4693, 4699, 1976). The enzyme is a unidirectional catalyst which brings about the reduction of NAD+ by NADPH + H+. The potential substrate NADP+ acts as an allosteric inhibitor, whereas other 2'-P nucleotides are allosteric activators. There are 1 catalytic site and 1 regulatory site per protomer. We now report the identification of a nonallosteric transhydrogenase isolated from a P. aeruginosa strain obtained from a hospital patient. No functional effector site appears to be present, and the catalytic site does not accommodate 2'-P nucleotides. Therefore, this enzyme form only catalyzes hydrogen transfers between NAD(H) and coenzyme analogs lacking the 2'-P group. It never constitutes the totality of the transhydrogenase activity detected in the bacterium, and its physiological role is still unclear. Its occurrence might be due to mutation and/or nutritional constraints.