Hexachlorobutadiene (HCBD) was identified as a component of the residue mixture of chlorinated hydrocarbons in fish from inland waters fed by the Rhine, particularly in the Ketelmeer. The residue level in fish is related to the fat content; no bioaccumulation in the food chain to higher trophic levels was observed. HCBD concentrations in the water of the Ketelmeer appeared to be three orders of magnitude lower than those found in aquatic animals.