Two bacterial strains, Bacillus subtilis and B. circulans, isolated from the gastrointestinal tracts of common carp, Cyprinus carpio L., and Mozambique tilapia, Oreochromis mossambicus (Peters, 1852), respectively were identified as potent cellulase producers. Both strains were cultured in tryptone soya broth for 24 h at 32 deg C. The effect of different production parameters, such as fermentation condition, moisture, pH, temperature, inoculum size, and nitrogen sources on cellulase production by the isolated bacterial strains were studied. Enzyme production in both the isolates increased in an optimum pH range of 7.0 to 7.5, maximum production was obtained at 40 deg C. Enzyme production increased for 96 h of fermentation in both strains, and decreased thereafter. The results may be help to better fish feed formulation incorporating plant ingredients, especially in the larval stages when the enzyme system is not efficient