Xylan is the most abundant noncellulosic carbon source present in wood and agricultural residues. Due the structural heterogeneity of xylan, its complete hydrolysis requires the action of several enzymes. The best characterized xylanases are endo-b-1,4-xylanases, which act on the main chain of xylan, and b-xylosidases, which hydrolyze xylobiose and short xylooligosaccharides liberating xylose. A Trichocladiun sp was isolated from corn straw and exhibited only traces of cellulase activity. One of the most important applications of xylanase is in the preableaching of kraft pulp. In this case it is very important the absence of cellulase activity, since it would weaken the strength properties of the pulp. The aim of this work was to purify and characterize an extracellular xylanase from this fungus. This fungus grew faster in xylose or xylan than in glucose as the only carbon source, and produced a large amount of xylanase activity when grown in semisolid medium of wheat straw. The purification procedure involved treatment with caulin, DEAE-cellulose and CM-cellulose chromatography. After the CM-cellulose step the specific activity was 13.5 U/mg protein and 22-fold purification was achieved, with a yield of 26%. Analysis in PAGE and SDS-PAGE showed a single protein band with a molecular mass of about 21 kDa. Maximal activity was achieved at pH 5.0-6.0 and at a temperature of 40ºC – 50ºC. The apparent Km and Vmax values were 2.7 mg and 17.3 U/mg protein. The purified enzyme was specific for xylan and did not hydrolyze, even at large incubation time, cellulose, starch, maltose, sucrose, raffinose, xylobiose, trehalose, p-nitrophenyl-b-D- xylopyranoside, p-nitrophenyl-b-D- glucopyranoside and p-nitrophenyl-b-D-arabinopyranoside. Analysis of the hydrolysis products on TLC showed the presence of xylopentaose, xylotetraose, and xylotriose at the first 15 minutes of reaction. After 60 minutes these xylooligosaccharides still accumulated, and after 90 minutes xylobiose was also detected in the reaction medium. These data strongly suggest that the enzyme is an endo-xylanase and the xylanolytic system of Trichocladium has potential use in the pulp and paper industry. Supported by CNPq and CAPES.