Bacillus circulans D1 is a good producer of extracellular thermostable xylanase. Xylanase production in different carbons sources was evaluated and the enzyme synthesis was induced by various carbon sources. It was found that D-maltose is the best inducer of the enzyme synthesis (7.05 U/mg dry biomass at 48h), while D-glucose and D-arabinose lead to the production of basal levels of xylanase. The crude enzyme solution is cellulase-free, even when the microorganism was cultivated in medium with D-cellobiose. When oat spelt xylan was supplemented with D-glucose, the repressive effect of this sugar on xylanase production was observed at 24 hours, only when used at 5.0 g/L, leading to a reduction of 60% on the enzyme production. On the other hand, when the xylan medium was supplemented with D-xylose at 3.0 or 5.0 g/L, this effect was more evident with 80 and 90% of reduction on the enzyme production, respectively. Unlike observed in xylan medium, glucose repressed xylanase production in maltose medium, leading to a reduction of 55% on the enzyme production at 24 hours of cultivation. Xylose, at 1.0 g/L, induced xylanase production on maltose medium. On this medium, the repressive effect of xylose, at 3.0 or 5.0 g/L, was less expressive when compared to its effect on xylan medium.