The routine use of chemotherapy to control bacterial diseases in aquatic populations has resulted in the development and spread of antibiotic resistance. The inclusion of immunostimulants in fish diets (functional diets) is one of the main strategies to solve this threat. This study aimed to analyse the intestinal microbiota of cultured European sea bass (Dicentrarchus labrax) fed two functional diets applying pyrosequencing of PCR-amplified 16S rRNA gene. Quality-filtered reads were assigned to family and genus taxonomic levels using the Ribosomal Database Project classifier. The autochthonous intestinal microbiota of sea bass consisted of two dominant bacterial genera: Dysgonomonas (Bacteroidetes) and Ralstonia (Betaproteobacteria), but effects of diet on this dominance were observed. In fact, the genus Dysgonomonas significantly decreased in samples from fish fed functional diets, recovering control levels at the end of the study. However, Ralstonia proportion significantly raised in samples from fish fed diet C and maintained this high level along the study period. The developed protocol could be used to study the composition of bacterial communities in the fish intestine under different nutritional and environmental conditions and its impact on infection, immune system and general fitness of fish.