The present study aimed to evaluate the influence of garlic essential oil (GEO) on the growth performance and morphometric variables of severum (Heros severus) post-larvae. A growth trial was performed using 150 severum post-larvae (1.6 ± 0.03 mg and 5.58 ± 0.1 mm), distributed in fifteen 1-L aquaria, with constant aeration. The fish were essayed in a completely randomized design with five treatments, using diets containing graded levels of GEO (0.00, 0.05, 0.50, 1.00 and 1.50 g kg-1) and three replicates. The diets were offered four times a day, until apparent satiation, for 30 days. At the end of the experiment, the fish were counted, weighed and measured, to evaluate growth performance and morphometric variables. Increased diet levels of GEO resulted in a linear reduction in growth performance parameters, except for the survival rate, weight and length uniformity of the batch, and morphometric variables which did not present significant differences. Elevated concentrations of dietary GEO may be detrimental to larvae and post-larvae of fish, which could be more sensitive to the potential damage that high concentrations of this essential oil may cause. Therefore, the inclusion of GEO in levels above 0.05 g kg-1 in diets for severum post-larvae is not recommended.