To reveal the catalytic mechanism of Thermus aspartate kinase, each of 29 amino acid residues that were highly conserved in the sequenced aspartate kinases, was replaced with alanine or leucine by PCR site-directed mutagenesis. Comparison of the kinetic parameters of these mutants with those of the wild-type aspartate kinase suggested that Thr47 was involved in binding aspartate and that Lys7 and Glu74 were involved in catalysis. Analysis of the effective concentrations of magnesium ion on the activity showed that the mutants with replacements at Ser41, Thr47, Asp154 and Asp182 required higher concentrations of magnesium ion. This suggests that these four residues play important roles in the binding of magnesium ions which are required for enzymatic activity.