Two isoenzymes of aspartokinase can be found in extracts of the differentiating bacterium Myxococcus xanthus. Aspartokinase I is repressed by L-lysine and feedback is inhibited by meso-diaminopimelate and by low concentrations of l-lysine. However, the inhibition by l-lysine is no longer observed at high concentration of this amino acid. Aspartokinase II is repressed and feedback inhibited specifically by l-threonine. Both enzymes are stimulated significantly by L-methionine and l-isoleucine; the effect is greater with aspartokinase I. The role of these enzymes in relation to growth conditions of the organism is discussed and a correlation with life cycle activity is indicated.