During the period 1977-1990 about 20 chlorobiphenyls (CBS) have been monitored in yellow eel (Anguilla anguilla) from inland waters of the Netherlands. CB levels found in eel from the rivers Rhine and Meuse are amongst the highest reported in fish. The high levels of lower chlorinated biphenyls in the river Roer, a tributary of the Meuse, could be attributed to the leakage of CBs from hydraulic systems in German mining areas. The discharge of CBs from unknown sources in the Netherlands could be identified. Yellow eel was shown to be a very practicable bio-indicator for the identification of spatial differences and temporal trends of CBs in surface waters.