During growth on glucose, Botrytis cinerea produced extracellular beta-(1,3)(1,6)-d-glucan (cinerean), which formed an adhering capsule and slime. After glucose was exhausted from the medium, cinereanase activity increased from <0.4 to 30 U/liter, effecting a striking loss in the viscosity of the culture. Cinerean was cleaved into glucose and gentiobiose. Gentiobiose was then hydrolyzed to glucose. While cinereanase activity was strongest in the culture supernatant, gentiobiase activity was located mainly in the cell wall fraction. The addition of extra glucose or cycloheximide prevented the cinerean degradation caused by an effect on cinereanase formation. Cinerean degradation was accompanied by microconidiation and sclerotium formation. B. cinerea was found to grow on cinerean with the latter as its single carbon and energy source. In this case, cinerean degradation occurred during hyphal growth, and no microconidiation or sclerotium formation was observed. Growth experiments with various carbon sources indicated that cinerean had a positive effect on the formation of cinerean-degrading enzymes.