Proteinaceous layers of the Thermococcus stetteri cell envelope were investigated and found to consist of regularly arrayed subunits 18 nm in diameter. According to the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, two major proteins were present. They were glycosylated and had molecular weights of 80,000 and 210,000. In addition to two external regular proteinaceous layers, cells of T. stetteri were found to have internal regular layers tightly attached to the cytoplasmic membrane. In the region of flagella attachment to the cell, polar membrane-like structures were found in the cytoplasm.