Mannanase belongs to hydrolytic family of enzymes which occupies a great commercial position because of their high stability under alkaline and acidic conditions. The current research was planned to isolate, screen and optimize mannanase production from Bacillus megaterium from decayed fruits. Microorganisms were isolated from fruits, soil and water samples obtained from different localities of Lahore city, Pakistan. Locally isolated microorganisms were screened for their mannanase producing characteristics on mineral salt agar medium pH 7. Six isolates were found to be mannanase producers by qualitative analysis. Three isolates (MBL-SP02, MBL-CW01 and MBL-GN02) produced maximum zone of hydrolysis with maximum activity ratio. These isolates (MBL-SP02, MBL-CW01 and MBL-GN02) produced maximum units of mannanase activity, i.e., 9.474, 7.364 and 3.568 U/ml in quantitative, respectively.These best producers were further selected for optimization of cultural conditions for mannanase production. They all produced maximum mannanase units in M1 medium after incubation of 48 hours. The MBL-CW01 was best mannanase producer at 35oC, pH 6.0 at 1% substrate concentration while MBL-GN02 produced maximum enzyme at 35oC, pH 4.0 with 0.5% substrate concentration. The MBL-SP02 produced maximum enzyme at 45oC, pH 7.0 with 2.0% substrate concentration. Among various carbon sources tested guar gum was the best producer of enzyme. Under these optimized conditions for each isolate, the best producers MBL-SP02, MBL-CW01 and MBL-GN02 produced maximum mannanase 22.07, 18.28 and 23.63 U/ml, respectively. MBL-GN02 showed maximum mannanase production and MBL-CW01 showed minimum mannanase production at optimized conditions of production.