Thermostable proteins from Thermus thermophilus HB8, an extremely thermophilic bacterium, were separated by two-dimensional gel electrophoresis. About 1200 spots were detected with silver staining on the gel between pH 3 and 10. According to the genome size of T. thermophilus, we consider that more than half of the proteins in the cell are visualized on a two-dimensional gel. Using comigrated standard marker proteins, the molecular weight and isoelectric point of each protein spot were calculated. The average molecular weight and isoelectric point values were estimated to be 30 000 and 5.2, respectively. The average size and isoelectric point of detected protein from T. thermophilus were smaller and more acidic than those from Escherichia coli. After the protein spots had been electroblotted onto a polyvinylidene difluoride membrane and stained with Coomassie Brilliant Blue, the N-terminal amino acid sequences were determined for about twenty protein spots. Few proteins had blocked N-termini. Some spots were identified as proteins whose sequences had been reported previously from T. thermophilus. Others had amino acid sequences homologous with those of various proteins from other organisms. The amino acid sequence information of this report will be useful for obtaining stable proteins and for identifying open reading frames determined from the genome DNA sequence. Considering its small genome size and protein stability, T. thermophilus will be an excellent candidate for studying the molecular biology of an autotrophic living cell at the atomic level.