The production of a highly thermostable mannanase by Rhodothermus marinus was increased 16.5-fold by optimising the concentrations of locust bean gum and yeast extract using central composite designs. The optimised medium and culture conditions yielded mannanase activity at 495 nkat ml(-1) (248 nkat mg(-1) protein). In addition, alpha-L-arabinofuranosidase, beta-xylanase, beta-xylosidase, beta-glucosidase, beta-mannosidase, alpha-galactosidase, beta-galactosidase and endoglucanase activities were detected at 32 nkat ml(-1), 30 nkat ml(-1), 16 nkat ml(-1), 15 nkat ml(-1), 0.1 nkat ml(-1), 1 nkat ml(-1), 0.5 nkat ml(-1) and 8 nkat ml(-1), respectively. No filter paper cellulase activity could be detected. The optimum pH of the mannanase was 5.0-6.5 and it showed high stability from pH 5 to 10 after 16 h incubation at 50 degrees C. The enzyme activity was maximum at 85 degrees C, with half lives of 45.3 h at 85 degrees C and 4.2 h at 90 degrees C. This is the first report on the production of such a high activity of extremely thermostable mannanase by an extreme thermophilic bacterium.