A thermophilic bacillus with minimal, optimal, and maximal growth temperatures of 40, 64.5, and 72 C, respectively, was isolated from soil. Biochemical and morphological studies place the isolate in group 1 of the classification of Walker and Wolf. After adaption to nitrate broth, the temperature coefficient for growth was found to be 20,400 cal/mol. When the temperature coefficient for growth of the isolate, psychrophilic bacteria, mesophilic bacteria, and a strain of Bacillus stearothermophilus are compared, there is no correlation with optimal temperature. The form of the Arrhenius equation, as used by some workers, is commented on.