Thermomyces lanuginosus RT9 was grown in a stirred and aerated reactor for the production of xylan-degrading enzymes and the kinetics of xylanase and beta-xylosidase secretion investigated. During each fermentation, k(L)a decreased with increasing mycelia concentration to the point of cellular disintegration. Although both agitation and aeration rates had significant influences on the growth rate as well as on the yields and rates of enzymes (xylanase and beta-xylosidase) release, the enzyme secretion kinetics showed a comparable pattern independent of the operating conditions. Enzyme secretion could be separated into a growth phase, during which extracellular xylanase was at low level, and an enzyme release phase, when there was a notable increase in the xylanase production rate accompanying cellular disintegration.