Plastic pollution of water bodies is a global environmental problem. Microplastics (MPs) can be ingested and accumulated by different types of organisms. Fish are particularly susceptible to ingest MPs due to their attractive coloration, buoyancy and resemblance to food. The aim of this study was to determine the direct effect of ingesting virgin low density polyethylene (LDPE)-MPs on the autochthonous intestinal microbiota of gilthead sea bream (Sparus aurata). Two sizes of LDPE-MPs (D2: 0.2-0.5 mm and D3: 0.5-1 mm) were introduced in the feed at environmentally relevant concentrations (0.12% w/w). Gilthead sea bream (140 g) were fed for 30 days with control (D1) or experimental diets (D2 and D3). Adherent microbiota was then extracted from anterior intestine and analyzed by Illumina sequencing of the V3-V4 region of the 16S rRNA.