Abstract.Parasitic cells resembling amoeba trophozoites were found histologically and ultrastructurally to be associated with severe proliferate gill disease and systemic infection often leading to extensive granulomatous tissue changes in the European catfish,Silurus glanisL. Infections with the normally free‐living amoeba leading to mortalities of up to 30% were considered to result from suboptimal water quality with heavy bacterial loading in a heated partial recirculation syste