Pyruvate carboxylase has been purified 400-fold from the thermophile, Bacillus stearothermophilus; it resembles pyruvate carboxylases purified from mesophilic organisms in its general kinetic and regulatory properties. The enzyme is virtually inactive in the absence of acetylcoenzyme A; this activating effect is antagonized by L-aspartate. Kinetic studies show that these two compounds act as allosteric effectors. ADP inhibits the enzyme activity competitively with ATP. Although the thermophile enzyme is appreciably more thermostable than similar mesophile enzymes, it is quite labile at the temperature at which the organism grows optimally, but can be stabilized by the two allosteric effectors and by some of the reactants.