Exo-1,4-beta-cellobiohydrolase (EC 3.2.1.91) was isolated from the culture supernatant of Thermotoga sp. strain FjSS3-B.1, an extremely thermophilic eubacterium that grows optimally at 80 degrees C. The enzyme was purified to homogeneity as determined by SDS/PAGE and has an Mr of 36,000. The enzyme is the most thermostable cellulase reported to date, with a half-life at 108 degrees C of 70 min in buffer. In a 40 min assay, maximal activity was recorded at 105 degrees C. Cellobiohydrolase from strain FjSS3-B.1 is active against amorphous cellulose and CM-cellulose but only effects limited hydrolysis of filter paper or Sigmacell 20. The only product identified by h.p.l.c. is the disaccharide cellobiose. The enzyme has a pH optimum around neutral and is stabilized by the presence of 0.8 M-NaCl.