Selective breeding of European sea bass (Dicentrarchus labrax) receives a growing interest, as the estimated heritability of growth is medium to high. In this study, we compared the offspring of four groups of sea bass sires, mated with the same wild dams: wild (W), first generation of domestication (D), first generation of mass selection for length (M), first generation of PROSPER-like selection for length (P). The comparison was done both in replicated tanks (separate rearing) and in mixed tanks (mixed rearing) where sire origins were recovered by genotyping of eight microsatellite markers. Weight, length and growth rate were measured from day 238 post-fertilization (69 g mean weight) to day 611 post-fertilization (390 g mean weight). Both in mixed and separate tanks. both selected groups (P, M) were larger than unselected groups (W D). No difference was seen at any time between Wand D, nor between M and P. The selection response estimate on weight was larger in mixed tanks when compared to separate tanks (+42% in mixed tanks, +23% in separate tanks at day 611), yielding realized heritability estimates of 0.60 and 0.34, respectively, and confirming the excellent potential of the species for growth improvement through selective breeding. Both selection response and the amplification effect between mixed and separate tanks decreased as rearing density increased. Our hypothesis is that selection response is magnified by competition in mixed tanks, while suboptimal rearing conditions lower the observed selection response, more in separate tanks (where selected thus larger fish are at a higher density than unselected ones) than in mixed tanks (where all fish experience the same density effects). (c) 2008 Elsevier B.V. All rights reserved.