1. NADP-+-specific isocitrate dehydrogenase [EC 220.127.116.11] was partially purified by about 440-fold from an extreme thermophile, Thermus flavus AT-62. 2. Remarkable thermostability of the enzyme was confirmed. The enzyme was not inactivated after 60 min at 70 degrees, and the activity was lost only slowly at 80 degrees. Above 90 degrees, however, rapid inactivation was observed. 3. The dehydrogenase was susceptible to concerted inhibition by oxaloacetate plus glyoxylate. In the presence of oxaloacetate plus glyoxylate (each 1 mM), 75 percent inhibition was observed. 4. The degree of inhibition of the enzyme by oxaloacetate plus glyoxylate decreased markedly above 60 degrees. The affinity of the enzyme for isocitrate and NADP-+ was also reduced markedly above 60 degrees. The activation energy calculated from Arrhenius plots below and above 60 degrees were 14,500 and 8,000 cal per mole, respectively. These observations suggest a possible conformation change of the enzyme protein at a transition temperature of 60 degrees, and the physiological significance of this in the adaptation of thermophiles to elevated temperatures is discussed.