Extracellular endo-1,4-β-xylanase synthesis in the yeast Cryptococcus albidus is largely inducible. During growth on wood xylans the yeast produces the enzyme in amounts two orders of magnitude greater than on other carbon sources, including xylose. The enzyme can be induced in washed glucose-grown cells by xylan and β-1,4-xylooligosaccharides. Among the oligosaccharides only xylobiose was not degraded extracellularly, therefore it appears to be the natural inducer of the enzyme. Xylobiose as a metabolisable inducer is effective at low concentrations and constant availability to cells. At high concentration of xylobiose the inductive effect is less pronounced because of catabolic repression by degradation products. Methyl β-D-xylopyranoside was found to serve as a non-utilizable inducer of β-xylanase. The enzyme induced by the glycoside appears to be identical with that produced by the cells during growth on xylan.