Abstract: Aspergillus niger MA was selected as the initial strain for β-mannanase production from a number of strains．After Co 60 mutagenesis of A．niger MA，a genetic stable and β-mannanase overproducing mutant A．niger MA-56 was obtained through primary and secondary screening．The activity of β-mannanase was improved about 64．78％ and reached 9．31× 10 4 U／g．The results of enzyme characteristics of β-mannanase showed that the optimum temperature was 70℃ and the optimum pH was 2．5—3．5．Moreover，β-mannanase was also stable at pH 2．5—9．0 at 60℃ ．Among different metal
ions，Cu2+ showed stronger inhibitory effect on the activity of β-mannanase.

Key words: β-mannanase, Aspergillus niger, Co 60 mutagenesis, enzyme characteristics