The soft-rot fungus Trichoderma reesei forms β-xylosidase (EC 3.2.1.37) activity during cultivation on xylan and xylose, but not on glucose. When mycelia precultivated on glycerol were washed and transferred to fresh medium without a carbon and nitrogen source, β-xylosidase formation was induced by xylan, xylobiose and xylose. A supply of 4 mm xylose and a pH of 2.5 provided optimal conditions for induction. β-Xylosidase accounted for the major portion of total extracellular protein under these conditions, and could be purified to physical homogeneity by a single anion exchange chromatography step. A recombinant strain of T. reesei that carries multiple copies of the homologous xylanase II-encoding gene has a six-fold increased xylanase activity, but forms comparable β-xylosidase activities. This shows that the rate of xylan hydrolysis has no effect on the induction of β-xylosidase. Methyl-β-d-xyloside inhibited β-xylosidase competitively and was a weak β-xylosidase inducer. The induction by xylobiose and xylan was strongly enhanced by the simultaneous addition of methyl-β-d-xylosidese and xylan or xylobiose. The results suggest that a slow supply of xylose is a trigger for β-xylosidase induction.