The effect of different carbon and nitrogen sources on the production of mannan-degrading enzymes, focussing on b-mannanase, by Aspergillus niger was investigated using shake flask culture. The bmannanase activity obtained during growth of A. niger on guar gum (GG, 1495 nkat mL À1 ) was much higher than those observed on other carbon substrates, locust bean gum (1148 nkat mL À1 ), a-cellulose (10.7 nkat mL À1 ), glucose (8.8 nkat mL À1 ) and carboxymethylcellulose (4.6 nkat mL À1 ). For fermentation using GG as a carbon source, bacteriological peptone gave the highest b-mannanase activity (1744 nkat mL À1 ) followed by peptone from meat (1168 nkat mL À1 ), yeast extract (817 nkat mL À1 ), ammonium sulphate (241 nkat mL À1 ), ammonium nitrate (113 nkat mL À1 ) and ammonium chloride (99 nkat mL À1 ) when used as a nitrogen source. The composition of bacteriological peptone and initial pH of the medium were further optimized using response surface methodology (RSM). Medium consisted of 21.3 g L À1 GG and 57 g L À1 peptone with initial culture pH of 5.5 was optimum for b-mannanase production (2063 nkat mL À1 ) by A. niger. The b-mannanase production obtained in this study using A. niger was significantly higher than those reported in the literature.