In many teleost species, fish assemblages are socially structured in pronounced dominance hierarchies. High-ranking individuals within a hierarchy are generally characterized primarily by being more aggressive than low-ranking individuals (subordinates) (Noakes 1980), and they grow markedly faster (Abbot & Dill 1989). From the aquaculturist’s point of view, dominance hierarchy formation may have several negative effects. The most obvious disadvantage is the differential growth rates among individuals in a group. It is generally accepted that chronic stress and social hierarchy alters the intestinal microbiota of endothermic animals (Tannock & Savage 1974; Tannock 1983), but to the best of our knowledge, no information exists on the effect of stress on intestinal microbiota of fish. The main objective of the current study was to investigate if hierarchy formation Arctic charr, Salvelinus alpinus (L.), affects the adherent intestinal microbiota of subordinate and dominant individuals. Hatchery-reared Arctic charr derived from the River Hals stock, Northern Norway, were used in the current study. The fry were reared on commercial