Twelve practical diets were formulated to contain four levels of Bacillus licheniformis (0.0, 0.24×106, 0.48×106 and 0.96×106 CFU g−1), respectively, with three yeast extract levels (0, 0.5% and 1%). Each diet was randomly assigned to duplicate groups of 50 Nile tilapia (Oreochromis niloticus) (5.99±0.03 g) for 12 weeks. Increasing dietary B. licheniformis levels in O. niloticus and yeast extract levels significantly (P