Abstract: Excessive aquaculture density may inhibit fish growth. To investigate the behavioral and physiological responses of largemouth bass (Micropterus salmoides) to density stress, we conducted an experiment with three densities Low (90 individuals·m⁻³), medium (110 individuals·m⁻³) and high (130 individuals·m⁻³). Then we determined the swimming behavior, enzyme activity and gene relative expression of energy metabolism in red and white muscles of M. salmoides after one week of density stress. The results show that the swimming frequency, tail-beat frequency and swimming speeds increased with density (P<0.05). Density did not significantly affect the gene expression and activities of anaerobic metabolic enzymes such as hexokinase (HK), phosphofructokinase (PFK) and lactate dehydrogenase (LDH) in red muscles (P>0.05). When the density increased from low to medium level, the gene relative expression of aerobic metabolism enzymes such as pyruvate dehydrogenase (PDH), citrate synthase (CS) and malate dehydrogenase (MDH) in red muscles increased significantly (P<0.05), so did the activities of these enzymes (P<0.05). When the density increased from low to high level, the gene relative expression levels of aerobic metabolism enzymes such as PDH, CS and MDH in red muscles, as well as anaerobic metabolism enzymes such as HK, PFK and LDH in white muscles increased significantly (P<0.05), so did the activities of these enzymes (P<0.05). It is indicated that density affects the energy metabolism of muscles by altering the swimming behavior of M. salmoides. Medium density mainly relies on aerobic metabolism of the red muscle for energy supply, while high density mainly relies on aerobic metabolism of the red muscle and anaerobic metabolism of the white muscle to participate in energy supply.