Biochemical and serological properties and antibiotic sensi tivity patterns for nine bacterial isolates from common carp (Cyprinus carpio L.) with carp erythrodermatitis in Hungary were studied. Also included were two other bacterial isolates from similar cyprinid fish ulcerative diseases -an unidentified bacterium from a crucian carp (Carassius carassius L.) in Hungary and an atypical Aeromonas salmonicida strain from a goldfish (Carassius auratus L.J in the United Sta tes. The carp erythrodermatitis isolates and the crucian carp isolate showed uniform biochemical reactions and were considered identical. These isolates could be distinguished from the goldfish A. salmonicida isolate by negative reactions in catalase and indole production tests; positive reactions in gelatinose and acetoin production tests; and failure to catabolize glycerol, mannitol and sucrose. Several phehotypic cha racteristics distinguished the carp erythrodermatitis isolates from each of the presently proposed subspecies of A. salmonicida. Antibiograms for all the bacterial strains tested showed few differences.Standard immunoelectrophoresis techniques revealed as many as five common antigens in most of the bacterial isolates. One antigen was lacking in one of the carp erythrodermatitis isolates and in the goldfish isolate. Line immunoelectrophoresis allowed direct comparison and identification of the antigens from different isolates. Since results of the serological study were preliminary, additional research is needed to better define the serological relationship among carp erythrodermatitis isolates and various strains of A. salmonicida.