Thermostable cellulase-free xylanase was produced by Streptomyces sp. Ab106 using agricultural waste, sugar cane bagasse, as the substrate at 50 degreesC and pH 7.0. The central composite face-centered experimental design was applied to evaluate the optimal agitation and aeration rates in a 5-l fermentor. The highest activity (16.0 +/- 0.5 IU/ml) was obtained at an aeration rate of 1 vvm and an agitation rate of 150 rpm (k(L)a=351 h(-1)). Using the repeated fed-batch cultivation technique, the maximum xylanase activity of 32 +/-1 IU/ml was obtained during the second cycle of repeated fed-batch culture.