The pH dependence of the kinetic parameters of the aspartase catalyzed reaction in the deamination direction was determined in order to obtain information about the chemical mechanism. The enzyme isolated from Hafnia aluei was metal ion independent at neutrnl pH and below. The V/K for aspartate was bell-shaped with the estimated pK values of 6.6 and 7.2. The maximum velocity for aspartate and the pH dependence of 1/K_i for succinate were also bell-shaped, giving pK values almost identical with those obtained for V/K_(aspartate). Two enzyme groups are necessary for binding of substrate and/or catalysis. An enzymatic group (a catalytic base) has been identified that must be deprotonated with a pK value near 6.6. Another enzyme group must be protonated for substrnte binding.