The coding sequence of a novel cellulolytic factor, swollenin, was isolated from the cellulolytic fungus Trichoderma pseudokoningii S38. The full-length swo2 gene encodes a protein of 494 amino acids with a calculated molecular mass of 51,393 Da, which includes a putative 22-amino-acid signal peptide. Sequence analysis revealed significant identity between isolated swollenin and that from Trichoderma reesei. The swollenin gene was further expressed and purified in T. reesei QM9414. The expressed swollenin protein was consequently purified by two-step ion exchange chromatography. The purified swollenin had subtle hydrolytic activities on xylan and yeast cell wall glucan, while no apparent activities on carboxymethy cellulose, cotton fiber, filter paper, or cellulose powder CF11 were observed. These results indicate that although swollenin maintains unidentified glycohydrolytic activities, it is inactive against beta-1,4-glycosidic bonds in cellulose. Its exact role in lignocellulose hydrolysis calls for further analysis.