The present communication describes the isolation of a virus from a single koi with altered gills. The isolate caused syncytia in cell lines developed from gills and brain of carp and in EPC cell cultures. The pathogen was sensitive to chloroform treatment and passed a 200 nm filter with only slight reduction of virus infectivity. Concerning the pathogenicity in koi no clear information was obtained in a preliminary challenge infection. Identification of the pathogen by chemical biochemical and physical characteristics is in progress.