This study evaluated productive parameters, whole-body composition, non-specific immune responses and pH and microbiota of digestive tract contents of red drum (Sciaenops ocellatus) fed diets supplemented with Ocimum americanum essential oil (OAEO) (0 – control, 0.25, 0.5, 1.0 and 2.0 g/kg diet). After 7 weeks no significant differences in productive parameters and whole-body composition were observed. Plasma and intestinal lysozyme measurements and pH of the stomach and intestine (6 h after feeding) did not show significant differences among groups. Intestinal microbial community in fish fed the basal and OAEO diets (all concentrations) were identical. However, red drum fed the diet with OAEO at 1.0 g/kg had significantly increased intraperitoneal fat deposition and stomach pH (2 h after feeding) and decreased superoxide ion production (NBT-test) compared to the control group. Hemolytic activity of the complement system increased in fish fed diets containing OAEO. Red blood cells from fish fed the lowest OAEO concentration (0.25 g/kg) showed significant lower fragility in erythrocyte osmotic fragility assay, but fish fed 0.5 and 1.0 g/kg showed significant higher erythrocyte fragility. Lysozyme measurement in the supernatant of stomach content was significantly higher in fish fed the diet supplemented at 0.5 g/kg. Based on these various results, OAEO at different supplementation levels did not influence growth performance and intestinal microbial community; however, the EO added to the diet showed effects on immunological responses of red drum.