A feeding trial was conducted to estimate the quantitative requirement of dietary methionine in largemouth bass［initial body weight (37.88±0.40) g］.Six isonitrogenous and isoenergetic diets(D1D6)were formulated to contain 44.39% crude protein,0.30% cystine,19.81 kJ/g gross energy using plant protein and crystalline amino acids as the primary protein source to simulate the indispensable amino acid profile of largemouth bass muscle except for methionine under investigation.Six graded levels of methionine(0.61%,0.83%,0.98%,1.21%,1.42%,and 1.59%,respectively)at about 0.2% increment were achieved by adding zeincoated crystalline Lmethionine at the expense of zeincoated crystalline glycine.Each diet was randomly assigned to triplicate tanks of 200L capacity and 18 tanks were connected to a freshwaterflowing and airaerated system in an aquiculture laboratory.In each tank,25 fish were fed by hand twice daily(8:30 and 15:30)to apparent satiation for 66 days.Water temperature was maintained at (27±1)℃ and dissolved oxygen content kept about 6 mg/L.Light cycle was controlled under 12∶12 h.At the end of the trial,data analysis showed that specific growth rate(SGR),feed efficiency rate(FER),protein efficiency rate(PER),protein deposition rate(PDR)increased with increasing dietary methionine levels from 0.61% to 1.21%(P<0.05)and thereafter showed a declining tendency.The crude protein contents of whole body and muscle were significantly affected(P<0.05)by dietary methionine levels,while moisture,crude lipid and crude ash contents showed no significant differences(P>0.05).The hepatosomatic index and viscerosomatic index of fish fed D1 diet(0.61% methionine)were significantly higher than those of fish fed the other diets(P<0.05).The serum lysozyme activity,phagocytosis activity and respiratory burst activity of head kidney leucocyte were significantly(P<0.05)affected by dietary methionine levels,which indicated dietary methionine level would also exert an effect on nonspecific immunity in largemouth bass.A quadratic regression analysis about specific growth rate(SGR)against dietary methinine level showed that the optimum dietary methinine requirement of largemouth bass was 1.22% of dry diet(corresponding to 2.75% of dietary crude protein containing 0.68% cystine).The present research may be helpful to formulating practical feed for largemouth bass.