We previously reported that lysine is synthesized through the alpha-aminoadipate pathway in Thermus thermophilus HB27 (T. Kosuge and T. Hoshino, FEMS Microbiol. Lett., 169, 361-367, 1998), which was the first report demonstrating the synthesis of lysine through the alpha-aminoadipate pathway in Bacteria. LYS20 and LYS4, which respectively encode homocitrate synthase and homoaconitate hydratase have already been identified as the lysine biosynthetic genes in T. thermophilus HB27. In the present work, we examined eight other Thermus strains for the existence of genes belonging to the alpha-aminoadipate pathway. BamHI- or Bg/II-digested total DNAs from the eight strains were analyzed by Southern hybridization using LYS20 or LYS4 as a DNA probe. DNA fragments that hybridized with one or both of the genes were detected in seven of the Thermus strains but not in T. ruber. The sizes of the fragments that hybridized with the LYS20 and LYS4: probes were the same among T. thermophilus HB27, T. thermophilus HB8, "T. caldophilus" GK24, and four "T. flavus" strains. For example, a similar 4.3-kb fragment was detected in each of the above seven strains. In this fragment, four open reading frames were found downstream of the LYS4 gene in T, thermophilus HB27. Gene disruption experiments revealed that three open reading frames are involved in lysine biosynthesis in T. thermophilus HB27. These results strongly suggest that the lysine biosynthetic gene cluster for the alpha-aminoadipate pathway is widely distributed in the genus Thermus.