The present investigation was carried out to analyze the screening and optimization of cellulase producing microbiome from the gut of marine fish Siganus javus. A major role of cellulases is the bioconversion of cellulosic waste materials into biofuels. Four different strains of intestinal microflora showed cellulase producing activity, of which Bacillus subtilis was observed to be the most potent strain in cellulase production. The strain showed high enzyme yield (88 U/ml) when glucose was used as the carbon source. Maximum enzyme production was achieved at pH 9.0 and at a temperature of 370C. The information generated through the present investigation contributes to the incorporation of these bacteria in commercial production of cellulase enzyme.