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• Investigation of size variability effects on D. labrax growth and feeding behavior. • Increased size variability favored productive traits. • Increased size variability favored milder feeding hierarchies establishment. Size heterogeneity can affect social interactions and growth performance of intensively reared fish species, while it creates difficulties in fish feeding as well as in harvesting fish of uniform size. The present study aims to investigate the effects of size variability on individual growth and feeding behavior of European seabass Dicentrarchus labrax. Fifty four fish (age 1+) were selected and distributed in nine tanks in groups of six, so as to form three triplicated different initial coefficient of mass variation, i.e. CV1: 1.6 % (80.5 ± 0.28 g), CV2: 11.5 % (82.3 ± 2.09 g) and CV3: 22.7 % (82.5 ± 4.14 g). Fish were individually marked with coloured beads in order to evaluate individual growth performance and behavior during feeding (video recordings). The experiment lasted for 75 days. Individual food consumption was used to rank individuals in the feeding hierarchy (Feeding Rank, FR). The individual assessment of growth and feeding behavior parameters showed that in all treatments a feeding hierarchy was established where fish of the highest FRs had the higher body mass and SGR, consumed the most part of food offered with little daily variation and made more efforts to claim food. This hierarchy was weaker in CV3 (lower correlation coefficients). Correlation coefficients between mean share of meal and coefficient of daily food consumption variation, which have been used as indicators of the strength of the social hierarchy, indicate CV3 as the condition creating the milder social environment and CV1 and CV2 as the ones leading to more intense social disturbance. In CV3 a greater percentage of fish showed satisfactory feed efficiency. On the other hand, in CV2 a greater percentage of fish lost weight, did not consume the ideal amount of food and had very low feed efficiency. CV1 presented intermediate results. Present results indicate that social hierarchies of different strength are present among treatments and may compromise fish welfare during their rearing. [ABSTRACT FROM AUTHOR]