Substrate degradation and production of related extracellular saccharifying activities were studied during a solid-stale culture of Trichoderma reesei on sugar beet pulp to characterize the substrate use : Different phases could be defined in this use. The first phase was characterized by the sole consumption of the free sucrose previously present in the substrate. The second phase corresponded to fungal degradation of the arabinose-containing polysaccharides, mainly arabinan, and consumption of part of the released arabinose. During the same period the consumption of sucrose was decreased and a production of different saccharifying activities appeared, After 90 h of culture, the glucose-containing polysaccharides,were preferentially degraded and used as the nutrient for mycelial growth; other polysaccharides were not, or only slowly, solubilized. Extracellular enzyme production increased and stabilized before the end of the culture at 165 h. alpha-L-Arabinofuranosidase was the main enzyme, produced at a rate of 433 IU g(-1) of dry fermented medium at the end of the culture (63 U mg(-1) of soluble protein), which was 70 times higher than the other polysaccharidases assayed. We found that this alpha-L-arabinofuranosidase was not able to degrade arabinogalactan, but seemed to be active on arabinan and xylan.