An extremely halotolerant mannan-degrading bacterium (strain NN) was isolated from the Great Salt Lake, Utah, USA. Strain NN grew at salinities from 0 to 20% NaCl with optimal growth at 0% NaCl. When grown on 0.2% (w/v) locust bean gum as the carbon source at 10% NaCl, both beta-mannanase and beta-mannosidase activities were produced. beta-Mannosidase activity was shown to be cell-associated, while at least 23% of the total beta-mannanase activity was extracellular. The optimum temperature and pH for beta-mannanase activity were 70 degrees C and 7.6, and for beta-mannosidase 25 degrees C and 7.0. The beta-mannanase system retained full activity after 24 h of incubation at 60 degrees C and 10% NaCl. beta-Mannanase activity was maximal at 1% NaCl and beta-mannosidase activity at 0.5% NaCl. Despite these low salinity optima, 50% and 100% respectively of the initial beta-mannanase and beta-mannosidase activities remained after 48 h of incubation at 20% NaCl, indicating a high degree of halostability. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis revealed the presence of at least eight different mannan-degrading proteins in the cell-free culture supernatant of cultures grown on locust bean gum.