Genetic parameters for somatic growth rate in sea bass were estimated in two experiments where 27 (year-class 1997) to 30 (year-class 1998) families of sea bass (3 dams mated with 9-10 sires according to a full factorial design) were raised mixed in the same tanks starting shortly before hatching (48 h post fertilization). Trials were performed under two temperature (experiment 1, 1997: group high temperature (HT) and group low temperature (LT)) and two density regimes (experiment 2, 1998: group high density (HD) and group low density (LD)) with a subset of the mating design (18 families) being reproduced in the two experiments. All the fish were genotyped at 3-6 microsatellite loci which allowed them to be assigned to the individual breeders used in the mating design. Heritability of (log-transfonned) body weight was estimated from the sire component of variance at a mean size of 20.0-21.6 cm (standard length (SL)) using a simple additive model and a restricted maximum likelihood algorithm. Estimates were 0.31 +/- 0.12 (group HT and group HD), 0.50 +/- 0.19 (group low temperature (LT)) and 0.60 +/- 0.22 (group low density (LD)), while an overall estimate using data from both year-classes was 0.29 +/- 0.13 indicating an important additive genetic component in growth rate of the sea bass in all conditions. Correlations of the estimated breeding values (EBV) of the sires between treatments and years were moderate (range = 0.01-0.51), suggesting the occurrence of genotype x environment effects. Growth was followed from the age of 341 days post fertilization (dpf (mean SL +/- S.D. = 16.0 +/- 1.9 cm) to the age of 818 dpf (mean SL = 32.6 +/- 3.1 cm) in the HT group. h(2) estimates tended to increase with the age of the fish (0.21 +/- 0.10 at 3 41 dpf to 0.56 +/- 0.20 at 818 dpf, r values=0.90, P=0.04). Genetic correlations among log weights recorded at various ages were high (range = 0.61-0.85, average = 0.70), indicating stability of the genetic values throughout the growth phase sampled and that growth as estimated at the earliest stage examined (341 dpf, 90 g) can be used as a predictor of later progeny growth (until 818 dpf, 737 g). (c) 2005 Elsevier B.V All rights reserved.