Two forms of the same commercial product (SORBIAL, Allonnes, France), one with live bacteria (PSA) and the other with heat-inactivated bacteria (PSI), containing a mixture of 2 strains of lactobacilli and their growth medium were tested as a diet complement for juvenile sea bass (Dicentrarchus labrax) during a 103-day experiment. In addition to zootechnical parameters (survival, growth, conformation), some effects on digestive metabolism were studied, including enzymatic, ultrastructural and microbial aspects. Microbial preparations improved survival rate. The ventral, dorsal and operculum malformations which usually occur in juveniles did not appear in those receiving PSA and PSI. Furthermore, they stimulated, but not constantly, trypsin and acid phosphatase activities. Intestinal ultrastructure showed an increase in the number of endocytosis vesicles at the apical pole of enterocytes in fishes receiving enrichments. Bacterial flora was not modified in terms of quantity, especially the lactic acid bacteria counts, which were not changed in fishes receiving live lactobacilli (PSA). The mode of action of these multiple beneficial effects appears complex and could be caused by different molecules inside the bacterial cell or excreted into their medium.