The effects of feeding two Bacillus spp. isolated from the intestine of the giant freshwater prawn on the growth of Giant Freshwater Prawns (Macrobrachium rosenbergii de Man) was examined. The isolated Bacillus KKU02 and Bacillus KKU03 (approximately 10(7) CFU mL(-1)) were mixed into commercial prawn feed (200 mL kg(-1)). After rearing shrimp with the bacteria in four feed treatments, (Bacillus KKU03, Bacillus KKU02, mixed culture and control groups) for 120 days, body length and weight of the prawns in mixed culture tanks were significantly higher (p = 0.05) than in the control tanks (7.48 cm and 3.32 g, vs 6.6 cm and 2.1 g, respectively). Both isolates were found to produce amylase and protease. The stabilities of the single Bacillus sp., mixed culture and commercial probiotic in the feeds were examined during storage at 4 degrees C and room temperature. The percentage viability of Bacillus KKU02, Bacillus KKU03 and mixed culture stored at room temperature declined dramatically to 2.54, 21.88 and 10.92% within 2 weeks, respectively. At 4 degrees C however, the percentage viability of the tested probiotics reduced slowly. The survival of the commercial probiotics was the same at both temperatures about 50% after 70 days' storage.