The present study demonstrates the enrichment, isolation and identification of cellulose degrading bacteria from the gut of cotton bollworm, Helicoverpa armigera. The isolate MD21 was identified as Klebsiella sp. MD21 (MG367463) based on 16S rRNA gene sequencing. The isolated bacterium, Klebsiella sp. MD21 secreted many hydrolytic enzymes particularly xylanase and p-glucosidase with highest activities of 276.71 +/- 0.98 and 78.45 +/- 0.10 IU/ml extract respectively. Scanning electron microscopy of treated filter paper and saw dust revealed structural alterations caused by Klebsiella sp. MD21 leading to hydrolysis of cellulosic waste into reducing sugars (2256 mu mol ml(-1) min(-1)) which could be used for the growth of bacteria. X-ray diffraction (XRD) pattern showed an increase in crystallinity index from 58.78% in native biomass to 60.10% of the treated biomass due to hydrolysis of amorphous regions present in the saw dust. Similarly, fourier transform infra-red (FTIR) spectroscopic study of the hydrolyzed saw dust indicated elimination of bands at wave numbers 718, 933 and 1533 cm(-1) which also represent the cellulose content. The present study demonstrates utility of H. armigera as a source for isolation of cellulolytic bacteria for use in biorefinery and pulp industries.