Isocitrate lyase in Escherichia coli and in Acinetobacter calcoaceticus is phosphorylated when the cells are grown with acetate as the sole carbon source in low-phosphate mineral salts medium containing P-32 inorganic phosphate. The level of P-32 incorporation into the enzyme in both microorganisms appears to be constant throughout the entire growth cycle. Further, the results of immunoblots and rocket immunoelectrophoresis suggest that the amount of isocitrate lyase protein, although at different levels in each microorganism, also remains constant throughout the growth cycle.