One 6-week feeding trial was conducted to determine the dietary protein requirement of juvenile giant croaker, Nibea japonica. Semipuri®ed diets were formulated with white ®sh meal to contain graded levels of protein ranging from 30% to 55% of dry diet. Each diet was fed to triplicate groups of juvenile giant croaker initially weighing 1.86 T 0.07 g ®sh ±1 (mean T SD) in a¯ow-through seawater system. Giant croaker were raised in aquaria with a water temperature of 16.4±20.0°C for 6 weeks. Weight gain and feed ef®ciency were best in ®sh fed diets containing 45% protein or more in the experiment. The protein ef®ciency ratio value was maximized in ®sh fed diets containing 45% crude protein, and declined in ®sh fed diets containing 50% crude protein and higher. Survival was independent of all diets excluding 30% crude protein. At elevated dietary protein level, wholebody moisture decreased and lipid increased (P < 0.05); however, all diets containing different protein levels produced similar condition factors, intraperitoneal fat, whole-body protein and ash. Hepatosomatic index and intestinosomatic index were maximized in ®sh fed diets containing 50% and 40% crude protein, respectively. Diets containing higher protein levels (45%±55%) produced signi®cantly higher muscle ratio values compared with the diets containing lower protein levels (30%± 40%), whereas ®sh fed all diets containing different protein levels had similar muscle ash, lipid, moisture and protein values. The optimal dietary protein requirement for juvenile giant croaker was deter-mined to be 45% of dry diet in natural seawater based on high weight gain, feed ef®ciency, protein ef®ciency ratio values and muscle ratio.