A third xylanase (Xyn III) from Trichoderma reesei PC-3-7 was purified to electrophoretic homogeneity by gel filtration and ion-exchange chromatographies. The enzyme had a molecular mass of 32 kDa, and its isoelectric point was 9.1. The pH optimum of Xyn III was 6.0, similar to that of Xyn II : another basic xylanase of T, reesei. The purified Xyn III showed high activity with birchwood xylan but no activity with cellulose and aryl glycoside. The hydrolysis of birchwood xylan by Xyn III produced mainly xylobiose, xylotriose and other xylooligosaccharides. The amino acid sequences of the N-terminus and internal peptides of Xyn III exhibited high homology with the family F xylanases, showing that they were distinct from those of Xyn I and Xyn II of T, reesei, which belong to family G. These results reveal that Xyn III is a new specific endoxylanase, differing from Xyn I and Xyn II in T.reesei. It is note worthy that this novel xylanase was induced only by cellulosic substrates and L-sorbose but not by xylan and its derivarives. Furthermore. T. reesei PC-3-7 produced Xyn III in quantity when grown on Avicel or lactose as a carbon source. while T. reesei QM9414 produced little or no Xyn III.