Diet supplementation with essential oil from sweet basil Ocimum basilicum (EOOB) can increase fi sh growth. So, this study aimed to evaluate the effect of EOOB in the diet on growth performance and plasmatic variables of pirarucu juveniles (Arapaima gigas) submitted to stressful condition (stocking density of 7.56 kg m -3 per tank and limited space). Four diets (in triplicates) were evaluated with increasing levels of EOOB (0.0 control; 0.5; 1.0; and 2.0 mL kg diet -1 ) over 48 days. Linalool was the major constituent of EOOB (54.19%). The addition of 2.0 mL EOOB kg diet -1 improved fi nal weight, weight gain, specifi c growth rate, condition factor and feed conversion ratio; it also decreased plasma urea levels and increased plasma albumin and total proteins levels. Plasma glucose, cortisol, and acid uric levels were not infl uenced by the addition of EOOB to the fi sh diet. In conclusion, the addition of 2.0 mL EOOBkg diet -1 is recommended for pirarucu juveniles, due to improved growth performance, and this supplementation did not compromise the homeostasis of fi sh rearing in a high stocking density.

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