The thermophilic eubacterium Rhodothermus marinus was cultivated in a fermentor and studied with respect to activities of induced xylanolytic enzymes. Growth in the fermentor on xylan occurred with a maximum specific growth rate of 0.43 h⁻¹ for a batch culture. The final cell concentration was 4 g cell dry weight (CDW)/l for cells grown on xylan compared to 2 g CDW/l for cells grown without xylan in the cultivation medium. At least two xylanolytic enzymes, endo-1,4-β-xylanase and xylan 1,4-β-xylosidase, were secreted into the culture medium when cells were cultivated on xylan. Of the three cellulolytic enzymes tested for activity, β-glucosidase activity was in the range of the xylanolytic enzyme activities whereas cellulose-1,4-β-cellobiosidase and cellulase activities were hardly detectable. The expression of endo-1,4-β-xylanase activities during cultivation indicates the existance of more than one xylanase in R. marinus. This is also observed in fractions from gel filtration. The xylanolytic enzymes are heat-stable. At 90�C and at pH 7.0 the half-life of the endo-1,4-β-xylanase was about 14 h and that of xylan 1,4-β-xylosidase was 45 min.