Under culture conditions, sex ratios in the European sea bass (Dicentrarchus labrax L.) are consistently skewed in favor of males. Moreover, no heteromorphic sex chromosomes have been shown for this species. This bias towards males could be due to a possible effect of environmental factors on sex differentiation, as has already been described in other fish species. To test this hypothesis, sexually undifferentiated sea bass were reared under two different thermal regimes or two different photoperiodic regimes, resulting in four groups: Group SP = short photoperiod (9 hours light [L]:15 hours dark [D]), Group LP = long photoperiod (15 L:9 D), both reared under natural temperature (24°C, average); Group LT = low temperature (15°C), Group HT = high temperature (25°C), both reared under natural photoperiod (13.5 L:10.75 D, average). Treatments lasted 80 days, from 57 to 137 days post fertilization, coinciding with the time when gonads are more sensitive to the action of exogenous sex steroids. The resulting sex proportions were determined by periodic histological examinations of the gonads during the first 2 years of age. No females were found in group LT at any sampling, whereas in the other groups, female proportions never reached values higher than 27%. Moreover, sex differentiation in group LT was delayed with respect to the other groups, as no fish completed sex differentiation until 16.4 months of age, compared to the other groups in which some individuals completed sex differentiation at 9 months of age. In addition, growth was significantly (P > 0.05) delayed in group LT with respect to the other three experimental groups and especially marked during the first year of life. Furthermore, the low values for the gonadosomatic index (GSI), together with the decrease of the number of spermatogenic males, denote a delay in gonadal maturation with respect to the rest of the experimental groups. The results of this study suggest that temperature can play an important role in the process of sex differentiation and growth of the sea bass. However, the mechanism(s) underlying this sexual lability remain unknown. J. Exp. Zool. 281:207–216, 1998. © 1998 Wiley‐Liss, Inc.