Radio-caesium is an important radionuclide released considering food and feed products. In aquatic environments caesium tends to accumulate in fish, both through its diet and its gills. This presentation discusses the caesium accumulation in fish living in estuaries. The aim of this work is to conclude on the importance of potential uptake routes: via the gills, diet or ingested with water. It is suggested that the magnitude of caesium accumulation in fish is related to waterborne potassium concentrations. Based on this theory, fish living in seawater should show a suppressed caesium accumulation compared to fish that stays in freshwater. However, to compensate for water losses, seawater fish are constantly drinking, adding an alternative route of entry for caesium. Apart from these two possible uptake routes of waterborne caesium, fish may take up caesium via their diet. Separate uptake-and-elimination studies were performed to measure the waterborne and dietary caesium uptake and elimination rate constants in fully acclimated Sea Bass at six different salinities (ranging from 1‰ to 35‰). Short-term uptake studies were performed as well to evaluate the effect of the waterborne potassium concentration. A toxicokinetic model evaluated several scenarios to conclude on the importance of each uptake route.