Abstract. – Gill epithelium of fish provides a dynamic ion transporting mechanism that consists of mitochondria rich cells (MRC), pavement cells (PC) and mucous cells (MC). In freshwater, MRC are the ones implicated in active uptake of Cl- induced by a Na+/K+-ATPase pump located in the basolateral part of the cell plasma membrane. Wels catfish Silurus glanis was held in closed experimental recirculation systems. Two groups were maintained for 51 days at a constant temperature of 26°C and two different salinities: freshwater (0.3 ‰) and brackish water (11 ‰) respectively. At the end of the experiment, gills were sampled for routine histology, TEM, SEM and immunocytochemistry in order to describe ultrastructural changes as well as localizing ATPase α-subunit activity induced by the environmental change. S. glanis gill structure shows typically the presence of MRC located in interlamellar epithelium both in freshwater and brackish environment, similar to the distribution observed in other freshwater-adapted species. However, in brackish water, dark MR cells go through a degeneration and necrosis process while small and compact immature cells with dark cytoplasm resembling β MR cells are frequently observed tightly enveloped by α MR cells. Evaluating overall appearance and functionality of S. glanis gill epithelium, we conclude that even though it shares many characteristics with other euryhaline fish, it also shows a peculiar process of differentiation and maturation of β MR cells enclosed by the cytoplasm of pale MR cells.