Reflecting the natural biology of mass spawning fish aquaculture production of fish larvae is often hampered by high and unpredictable mortality rates. The present study aimed to enhance larval performance and immunityvia the oral administration of an immunomodulator, A-glucan(MacroGard') in turbot(Scophthalmus maximus). Rotifers(Brachionus plicatiilis) were incubated with or without yeast B- 1,3/1,6-glucan in form of MacroGard at a concentration of 0.5 g/L Rotifers were fed to first feeding turbot larvae once a day. From day 13 dph onwards all tanks were additionally fed untreated Artemia sp nauplii(1 nauplius ml/L). Daily mortality was monitored and larvae were sampled at 11 and 24 dph for expression of 30 genes microbiota analysis, trypsin activity and size measurements. Along with the feeding of B-glucan daily mortality was significantly reduced by ca. 15% and an alteration of the larval microbiota was observed. At 11 dph gene expression of trypsin and chymotrypsin was elevated in the MacroGard fed fish, which resulted in heightened tryptic enzyme activity. No effect on genes encoding antioxidative proteins was observed, whilst the immune response was clearly modulated by B-glucan. At 11 dph complement component c3 was elevated whilst cytokines, antimicrobial peptides, toll like re- ceptor 3 and heat shock protein 70 were not affected. At the later time point(24 dph) an anti inflammatory effect in form of a down-regulation of hsp 70, tnf-ca and il-1B was observed.