This study evaluated the effect of initial larval stocking density and microalgae concentration in the diet on the growth and the number of competent larvae (number of live larvae at the end of the experiment retained on a sieve with mesh size of 210 µm) of Perna perna in a water recirculation system (RAS). The experiment was conducted using a completely randomized design and factorial scheme, with one factor being the food concentration at three levels (low: between 1.26 to 2.19, medium: between 2.36 to 3.94 and high: 5.03 to 7.29 x 104 cells mL⁻¹) and the second factor being the larval culture density, also at three levels (20, 80 and 200 larvae mL⁻¹), with 3 replications in each treatment. Larvae fed with the diet with low and high concentrations did not reach competence (210 µm) in 16 days. However, for larvae fed with the medium concentration (2.36 to 3.94 x 104 cells mL⁻¹), the average percentage of competent larvae, on day 16, was 21.8% ± 7.45, 27.4% ± 9.91 and 0.80% ± 0.26 at the densities 20, 80 and 200 larvae mL-1, respectively.