The effects of dietary administration of probiotic Pdp11, a bacterial probiotic strain from the Alteromonadaceae family, on growth and stress tolerance to high stocking density was analyzed in juvenile gilthead sea bream Sparus auratus. In the first trial, juvenile specimens were fed during 116 days with control feed or diets supplemented with Pdp11, and growth, serum immunological parameters as well as plasmatic, hepatic and muscular metabolic parameters were assessed. Growth performance improved in group receiving probiotic compared to control group. No differences were observed for the assessed immunological or metabolic parameters. In the second trial, specimens from both groups from the first experiment were submitted to the following conditions for 15 days: i) group fed with control food and either under low density (3 kg/m 3), or high density (30 kg/m 3) and ii) group fed with probiotic supplemented feed and either under low density, or high density. Our results indicated that administration of probiotic Pdp11 in the diet promoted growth and improved stress tolerance under high stocking density, suggesting its use will be beneficial for the aquaculture industry of this species.