Aim: Isolation of cellulase producing fungi and increasing cellulase production using novel mutations. Methods and Results: Cellulase-producing fungi were isolated from different soil samples using enriched Mandels cellulose agar, which is a selective media and seven different fungi were selected in the screening programme. These organisms were tested for cellulase production and two potent strains were identified. Two methods of mutations for strain improvement were employed to these strains. (1) Germinating fungal spore suspension was treated with 0.1 and 0.2 mg ml(-1) of 1-methyl-3-nitro-1-nitrosoguanidine (MNNG), ethidium bromide (EtBr) and u.v. for 30 min and 1 h duration and plated on selective media with and with out amphotericin B. (2) Mutagens (EtBr and MNNG) were incorporated in the selective media in sublethal concentration (5 mug ml(-1)) along with antifungal antibiotic (amphotericin B 2 mug ml(-1)). Second method yielded maximum cellulase-producing mutants, which are also stable for cellulase production and are more potent than the mutants obtained by the first method. Conclusions: Mutations using sublethal concentrations of mutagen for a prolonged period of growth has yielded mutants, which can produce more cellulase. Significance and Impact of the Study: This new method could be applied to obtain potent fungal mutants for more enzymes production.