Effect of lignocellulosic medium supplemented with selected easily-metabolised carbon sources on microbial xylanase production was assessed. A newly isolated oil-palm-waste-domesticated bio-agent-producing fungus, identified based on rRNA analysis as T. asperellum USM SD4 was used as a representative organism. The potential of T. asperellum for enhanced xylanase production was evaluated by the statistical optimization of important cultural parameters via response surface methodology (RSM). T. asperellum showed optimum xylanase activity at pH 7; temperature 27 °C; moisture content 4 mL growth medium (gm): 1 gram dried substrate (gds) and inoculum size 2 x 106 spores/mL. Xylanase activity (2,337 IU/gds) attained in this study was far higher than ever reported for T. asperellum. Using the set of optimum conditions, the mixture of supplementary sugars to the lignocellulosic medium initiated xylanase repression in a concentration-independent manner. However, the degree of repression depended on the nature and type of respectively added sugar. The repressive effect exerted by monosaccharides (xylose, glucose, and fructose) was greater than exerted by either of dimeric (cellobiose and sucrose) or polymeric (xylan) sugars. Of all added substrates, xylan exerted the least repressive effect. Using xylose as a representative sugar, mechanism of xylanase repression was decisively explained and supported with experimental data.