The dnaK locus of Methanosarcina mazei S-6, a mesophilic organism of the phylogenetic domain Archaea, contains the heat-shock genes 5'-grpE-dnaK-dnaJ-3'. Parameters known to affect the response of these genes in organisms of the other two domains, Bacteria and Eucarya, were tested to determine their effects on the archaeal homologs. The mRNA from the three genes increased after heat shock more in lamina than in single cells (these S-6 morphologic stages can be grown in the same substrate), Single cells in early stationary phase showed the highest levels of dnaK mRNA after heat shock, as compared with cells in exponential, or in late stationary, phase. The dnaK mRNA always had the size of a monocistronic transcript, dnaK was also found in the thermophile Methanosarcina thermophila TM-I, and its response to heat shock showed distinctive characteristics. However, dnaK was not revealed in other archaea: three hyperthermophiles (Methanothermus fervidus, Methanococcus jannaschii, and Sulfolobus sp,), and one mesophilic methanogen (Methanospirillum hungateii).