Substrate (wheat bran) particle degradation studies were conducted in solid state fermentation employing Aspergillus niger CFTRI 1105. Changes in the substrate weight and variations in the microscopic appearance of the particle were made during the course of fermentation. The sequential release of extracellular enzymes by the fungus recorded at different fermentation times. The maximum reduction in size and total weight of the substrate particle and the release of all the major hydrolytic enzymes were observed after 36 h of fermentation. A mathematical model was proposed for the degradation of the substrate particles during fermentation. Model predictions coincide with the experimental observations of the extent of reduction in particle size.