The extremely radioresistant bacterium Deinococcus radiodurans is evolutionarily closely related to the extremely thermophilic bacterium Thermus thermophilus. These bacteria have a single gene encoding an aspartate kinase (AK) that catalyzes the phosphorylation of L-aspartate. T. thermophilus has an aminoadipate pathway for lysine biosynthesis that does not use AK for lysine biosynthesis. Phylogenetic analysis in this study indicated that D. radiodurans AK has a different protein structure and a different evolutionary history from T. thermophilus AK. Disruption analysis of D. radiodurans AK indicated that D. radiodurans AK was not used for lysine biosynthesis but for threonine and methionine biosyntheses. A D. radiodurans AK disruption mutant exhibited a phenotype similar to a T. thermophilus AK disruption mutant, which indicates that these two AKs have different evolutionary origins, though their functions are not different.