Trichoderma reesei was co-cultured with either Aspergillus niger or A. phoenicis in solid substrate fermentation on sugar cane bagasse for xylanase production. When soymeal was used as nitrogen supplement, optimal activities of cellulase (14-15 IU/g dry wt) and xylanase (2,600-2,800 IU/g dry wt) were attained by both mixed culture systems in 72 h of fermentation, corresponding to xylanase volumetric productivities of 5,500-5,900 IU/L. h.