The studies on optimisation of citric acid biosynthesis by solid-state fermentation of sugar- beet pulp as the major material have shown that not only mono- and disaccharides initially present in the material but also the products of cellulose and hemicellulose hydrolyses were the carbon sources. Therefore, in parallel with optimisation of citric acid production, the studies were undertaken on the activity of cellulases and dynamics of their synthesis by Aspergillus niger, the strain used in the process. The fermentations were carried out by non-mixed system using 1 L beakers and by mixed system using 4.5 L rotating drum bioreactor. The activity of CM-cellulase and xylanases for the non-mixed system increased to about 22.0 IU/g and that of FP-ase stayed on the level of about 4 IU/g dry matter, whereas for the mixed system the CM-cellulase activity increased to about 35 IU/g dry matter on day five and that of FP-ase stayed on a similar level of about 5 IU/g dry matter. The content of reducing compounds increased by almost four times (to 390 g/kg dry matter) on the second day of fermentation and decreased to about 140 g/kg dry matter on the fifth day of fermentation. Glucose was utilised most rapidly among monosaccharides, being followed by arabinose and in part by mannose.