In this report, we isolated and identified a strain of Flavobacterium columnare (named Pf1 strain) from a diseased yellow Chinese catfish, Pelteobagrus fulvidraco. Subsequently, the drug sensitive tests and the pathogenicity of Pf1 on yellow Chinese catfish and mandarin fish (Siniperca chuatsi) were carried out and the results showed that Pf1 was pathogenic. Pf1 infection could cause necrosis and inflammation of several tissues, especially the liver, trunk kidney and gill in mandarin fish. In the infected mandarin fish, liver cells were swollen, vacuolar and necrotic. Necrosis of renal tubular and extensive inflammatory cells infiltration were observed. Gill lamellae capillaries were swollen and hyperemic, and respiratory epithelia were swollen and degenerated. The lesioned gill lead to the higher breathing rate, slower swimming, and eventually mass mortality of the infected mandarin fish.