Recent results have demonstrated that live yeast in diets improved gut maturation in sea bass larvae. In this study, we tested the absence and two levels of live yeast in sea bass larvae diets. Specimens were fed from time of mouth opening to 37 days after hatching a diet of 0%, 1.1%, or 5.7% wet weight of live yeast (Debaryomyces hansenii CBS 8339). Yeast incorporation improved survival 10%, and reduced malformed larvae. In groups fed 1.1% yeast, only 1% of larvae were malformed, compared to 14% in the control group. Final mean weight in groups fed 1.1% yeast was twice that of other groups. Activities and concentrations of mRNA trypsin and lipase were higher in the two groups fed yeast than in the control group, whereas activity and concentration of mRNA amylase were lower. This suggests that the pancreas matured faster in the two groups fed yeast. Activities of intestinal enzymes alkaline phosphatase, aminopeptidase N, and maltase in the group fed 1.1% yeast were higher than those in the two other groups, revealing earlier development of intestinal digestion. The best results were obtained with the diet containing 1.1% yeast cell biomass, corresponding approximately to 10(6) CFU g(-1) in the diet and 1.1 x 10(4) CFU per larva 30 days after hatching. The dose-dependent effect of yeast on rearing performance could be attributed to the amount of polyamines secreted by live yeast in the gut lumen of larvae.