The effect of easily metabolizable sugars (glucose, xylose, fructose, maltose, cellobiose and lactose) on xylanase production by Aspergillus tamarii in solid-state fermentation (SSF) was studied using wheat bran, corn cob and sugar cane bagasse as substrate. The addition of different sugars at a concentration of 1%, to sugar cane bagasse or corn cob media caused sever catabolic repression. In contrast, wheat bran systems were resistant to catabolic repression even at high concentrations of glucose (10%). In wheat bran cultures, glucose uptake was very efficient and less than in: of initial glucose concentration was present in the culture medium after 4 days of cultivation. In corn cob and sugar cane bagasse cultures, glucose: uptake was slower and more than 60% of the initial glucose concentration was present in the culture medium after 4 days of cultivation. These results demonstrate: the importance of the nature of the substrate used in solid state cultures to obtain a system resistant to catabolic repression.