Abstract: An 89-day growth trial was undertaken to evaluate effects of essential amino acid (EAA) supplementation to diets with lower protein levels on growth performance, body composition and immunological parameters in largemouth bass (Micropterus salmoides). Seven isoenergetic diets were formulated according to the EAA profile of whole body of largemouth bass. Four diets of them contained crude protein concentrations of 45%, 40%, 35% and 30%, respectively (designated as 45CP, 40CP, 35CP and 30CP). The other three diets (designated as 40AA, 35AA and 30AA) were formulated by adding EAA to three lower protein diets of 40CP, 35CP and 30CP to make their EAA levels consistent with that of 45CP (used as a control). Triplicate groups of thirty juvenile largemouth bass with initial body weight of (10.130.01) g were reared in an indoor water-circulating system. It was showed by one-way ANOVA that growth performance, feed utilization, crude protein concentrations of whole body and muscle, immunological index and survival rate declined significantly among fish fed the diets with a reduction of dietary protein from 45% to 30% (P0.05). Feed efficiency and protein retention in fish fed 35AA and 30AA were significantly higher than those of 35CP- and 30CP-fed fish (P0.05), respectively. There were no significances in serum lysozyme activity and serum complement activity between 40AA-fed fish and 45CP-fed fish (P0.05). Respiratory burst activity of head kidney leukocytes with 35AA-fed fish and 30AA-fed fish was significantly higher (P0.05), compared to those of 35CP and 30CP, respectively. Crude protein concentrations of whole-body and condition factor of 30AA-fed fish were significantly higher than those 30CP-fed fish (P0.05). Among all the treatments, moisture and ash of whole body and muscle of tested fish showed no significant differences (P0.05). It was suggested by regression analysis that the effects of supplementing EAA to lower protein diets on SGR, FE and PR in juvenile largemouth bass were positively related with increased levels of dietary protein rather than those of dietary EAA. This study indicates that crystalline EAA supplemented to lower protein diets for juvenile largemouth bass can cause improvement in growth performance, feed efficiency and immunological index but the improvement is inferior to what is caused by the same levels of EAA deriving from proteins.