Following a recent unpublished trial which determined the optimal dietary supplemental γ-Aminobutyric acid (GABA) level to be 145 mg kg-1, an 8-week feeding trial was conducted to determine the effects of GABA approximating optimal supplementation at four different levels on juvenile olive flounder reared at both optimal and high stocking densities . A total of 1,080 fish with an average weight of 5.66 ± 0.02 g(mean±SD) were randomly stocked at two densities, 30 fish and 60 fish into 24, 40 L , 0.153 m2 seawater tanks which resulted in 1.11 and 2.22 kg/m2 respectively . Each t ank at both densities were assigned to triplicates of four dietary treatment groups; a basal diet without supplemental GABA (CON) and three other diets prepared by adding 150 (GAB150) , 200 (GAB200), and 250 mg kg-1 (GAB250 ) GABA to the CON diet. Parameters including growth performance, non-specific immune responses , disease resistance, enzyme activity, histology, and genetic expression are currently under analysis. The final r esults of this trial will be reported in conference.