Several protein sources were formulated within isoproteic and isocaloric diets and fed at the same level to seabass fingerlings. The diets resulted in different growth rates which could not be explained either by digestibility of by a deficiency or imbalance of essential amino acids. The measurement of feed intake and gastrointestinal transit was performed using a new fast and accurate technique, but did not shed any light to explain the growth differences observed; although it did allow a calculation of the maximal daily ration for the seabass fingerlings. During these experiments, feeding frequency appeared to be another factor influencing growth; the sequential distribution of the ration inducing a faster growth than continuous feeding of the same amount of food. However, the differences in growth could be explained by mechanisms concerning protein metabolism due to differences in protein quality and method of feeding. For example, protein quality influences protein accretion by acting on the retention of synthesised protein (decreasing its degradation), rather than on the synthesis which is bound to the body RNA content; the ribosomal activity remaining unchanged. Moreover, under the action of sequential feed distribution, protein degradation increases while synthesis is stimulated under the effect of an increase of RNA content and ribosome activity.