A total of 821 sera from salmonids, and( Salmo gairdneri , Salmo trutta ), and( Oncorhynchus tshawytscha )were examined for antibodies against the causative agent of BKD (Renibacterium salmoninarum). As serological tests microtiter agglutination and a hemagglutination-inhibition (HI) - test were used. Positive serum samples were detected in 28 out of the 29 trout farms investigated. Sera of only one farm had no KD-antibodies. A higher incidence of positive reactants with increased serum titres was found in farms, where clinical signs or pathological evidence of BKD were noted.