Free D-amino acid content in some archaea was investigated and D-forms of several amino acids were found in them. in the acidothermophilic archaeon, Thermoplasma acidophilum, the proportion of D-aspartate (D-Asp) to total Asp was as high as 39.7%, Crude extracts of Thermoplasma acidophilum had Asp-specific racemase activity that was pyridoxal 5'-phosphate (PLP)-dependent. The relative insensitivity to a SH-modifying reagent distinguished this activity from those of the PLP-independent Asp racemases found in other hyperthermophilic archaea (Matsumoto, M,, ct at, J. Bacteriol. 181, 6560-6563 1999), Thus, high levels of D-Asp should be produced by a new type(s) of Asp-specific racemase in Thermoplasma acidophilum, although the function of D-Asp in this archaeon remains unknown.