Five different diets were offered simultaneously to each of three different groups of fish under self feeding conditions. Diets differed only in methionine content (0.30, 0.65, 1.00, 1.35 and 1.70% of diet). Fish groups differed in weight (group A, 0.2 g; group B, 2.0 g; group C, 200 g mean weight). Measure of food demand showed that group A refused feeding and declined in vitality and health; group B showed a net preference for 1.35% methionine (considered as optimum); group C was more influenced by spatial preferences. In a second experimental period, the following modifications were introduced: commercial food for sea bass (Aqualim dry pellet) was offered to group A in addition to the five initial diets; for groups B and C changes in spatial distribution of self feeders were introduced. The fish in group A reacted instantly to the introduction of the commercial food which they ate preferentially. Fish of groups B and C maintained behaviour similar to that of the previous period. The authors conclude that sea bass can detect and discriminate small doses of methionine, showing preferences easily demonstrable with the self-feeding method.