The rise in world industrialization and the cost of importing enzyme by local industries have led to arise in the search for novel and native enzyme producing microorganisms. Cellulase is an enzyme that catalyzes the breaking down of carbon chains in cellulose and hemicellulose, this research therefore aimed at studying fungal cellulase produced by Penicillium expansum grown on malus domestica (apple fruits). Fresh apple fruit was allowed to deteriorate under laboratory condition until there was visible mould growth. The mould with desired features of the organism of interest was subcultured by direct plating on PDA plates to which 10 % streptomycin has been added to prevent bacterial contaminants. The plates were incubated at 28±2 0C for 7 days until a visible mass of blue mycelia appear. The isolate was further subcultured onto freshly prepared media until pure culture was obtained. Characterization and identification of isolate were done using macroscopy and microscopy techniques. The isolate was re-inoculated into healthy apple fruits and the fruits were incubated at temperature of 28±2 oC for 8 days. Cellulolytic activity was examined every day throughout the incubation period. Crude enzyme was extracted each day using standard methods. Carboxyl methyl cellulose was used as standard for the crude cellulase activity assay after extraction from the infected apple fruits using Dinitrosalicylic acid (DNSA). Culture parameters like pH and temperature were also optimized to determine their effect on cellulolytic activity of the fungus. Cellulase activity was defined as the amount of glucose produced in μmol/mg/min under the assay condition. The highest cellulase activity of 86.84±0.52 μmol/mg/min was observed on day 6 of incubation at 28±2 oC and at pH 7. In conclusion, it is evident from this research that P. expansum isolated could be used as potential novel organism for industrial production of cellulase under optimized fermentation conditions.