We exposed juvenile European sea bass (Dicentrarchus labrax) to chemical pollutants and to polypropylene MPs contaminated or not with a blend of chemical pollutants, by adding them to the fish diet. The aim was to determine the individual and combined effects of dietary MPs and chemical pollutants on gut microbiota. Four different diets were formulated using as basis a commercial pellet diet that represented the control Diet A. A mix of three chemical pollutants at high concentration, MPs, or MPs-contaminated by low concentration of chemical pollutants mix were added to the control Diet A to produce Diet B, C, and D, respectively. Fish were fed ad libitum with four different diets in triplicate for 60 days.