The serological characteristics of the strains of Cytophaga psychrophila isolated from different fishes in different areas were compared by the microtiter method. Included were 3 strains from cobo salmon (Oncorhynchus kisutch) in Miyagi Prefecture, 4 from rainbow trout (Oncorhynchus mykiss) in Nagano and Tokyo Pref., 6 from ayu (Plecoglossus altivelis) in Tokushima and Wakayama Pref. In addition to these Japanese strains, 3 strains from coho salmon in USA were used as references. Three antisera were prepared with the type strain NCMB1947 from coho salmon in USA, FPC828 from coho salmon in Miyagi, and FPC840 from ayu in Tokushima. In the comparison of C. psychrophila strains by using these antisera, all were positively agglutinated by each of the antisera, indicating the presence of common antigen(s). On the basis of the absorption analysis with thermostable antigens, two distinct serotypes (O-1 and O-2) were recognized. The strains from coho salmon in Japan and USA belonged to serotype O-1. Serotype O-2 occurred only in Japan, and was distributed among rainbow trout as well as ayu.
