Sea bass, a fish species characterized by its dualistic feeding pattern, was investigated to study the synchronizing effect of light and food on the demand-feeding rhythm. Nocturnal and diurnal sea bass, both in groups and individually, were exposed to restricted-feeding (RF) and light-dark (LD) cycles of different periods. The phase relationship between both zeitgebers was also studied. The results show that food-demand rhythms synchronize to periodic food availability under constant light conditions (DD) and that there is a partial coupling between food-entrained and light-entrained activity under conflicting zeitgebers (LD 13:13 h and RF 4:20 h), suggesting the existence of a feeding entrainable oscillator (FEO) in addition to the master light entrainable oscillator (LEO). In some cases, food availability restricted to the light or dark phase contrary to that of the previous feeding phase changed a diurnal feeding pattern into nocturnal and viceversa, suggesting that food can be one of the switching factors that decides whether the circadian system of sea bass is nocturnal or diurnal. However, the fact that the feeding pattern of some fish was unrelated with the phase in which food was available suggests that other internal and/or external factors could be involved in the temporal flexibility of sea bass.