Abstract: In recent years, dermal ulceration disease is one of the most serious diseases on cultured Japanese eels (AnguillaObservation on the morphology and life history of strain Js80122 cultured on slide and cover glass showed that strainJs80122 was the filamentous fungi and could grow well at 15℃, 20℃ and 25℃. Its hyphae were non-aseptate withplentiful branches, the reproductive modes were asexual and sexual. The asexual structure of Js80122 could be wellproduced at 20℃ or 25℃, the sexual structure could not be produced at those temperatures hereinbefore, even culturedfor 4 weeks, but it could be produced at 15℃ when the strain was cultured for more than 3 weeks. Zoosporangium wasclavate or spindly and renewed by internal or cymose proliferation, zoospores were diplanetic. Every oogonium couldform more than one oospore, which were smooth. Flagella stain with AgNO3 revealed that there were two flagella withalmost equal length on the zoospore. According to the characteristics of morphology and life history, such as the modalityof the hyphae, the reproductive modes, the modality and the mode of movement of the zoospores, the amount andmodality of the oospores, etc, strain Js80122 was identified as Saprolegniaceae, Protoachlya Coker, Protoachlya paradoxa.Epidemiological investigation showed that the epidemic temperature of the dermal ulceration disease was under20℃, but it was nonoccurrence when the aquatic temperature exceeded to 23℃, and it could not be prevented and controlledby using multifarious disinfectors or antibacterial medicaments in breed aquatics of Japanese eels, contrarily, itwould be more serious if the antibacterial medicaments were used in the beginning. Additionally, the fungal hyphaecould be easily observed under the microscope even at the initial stage of the disease, when the derma of the diseasedeels was unworn and the muscle had not been rotted. Strain Js80122 could cause dermal ulceration disease even theexperimental eels were infected with spores alone as already noted. According to the results of epidemiological investigationand this study, it could be confirmed that the primary pathogen of dermal ulceration disease on cultured Japaneseeels was not any pathogenic bacteria but fungi, in the concrete, it was Protoachlya paradoxa, the bacteria caused secondaryinfection, and they brought on nothing more than the worse illness.japonica), which occurs widely, spreads quickly and has caused great expense through the winter. In the beginning,the grume of the diseased eels are hyperplastic and then broken off. Following the evolvement of the illness, thederma is bareness and the muscle is necrotic. Eventually, the derma and the muscle are rotted.The isolation and identification of the fungal pathogen of this disease was studied. The fungal strain Js80122 wasisolated from diseased eels by cultured on kernels of wheat. Strain Js80122 was confirmed as the causative pathogen ofthe dermal ulceration disease by artificial infection tests, which lasted for 2 weeks. In these tests, healthy eels were infectedwith four kinds of different methods, namely, injected with spores (0.3 mL/ind.) on dorsal muscle, marinated with3 dish of culture after ravaged, daubed with spores(1.0×108 cell/mL)on wound and injected with spores and bacteria (0.3mL/ind.) on dorsal muscle. The results showed that dermal ulceration disease could be recurred well on the experimentaleels by any infection method; the symptom and process were similar to the naturally diseased eels. During the course ofthe artificial infection tests, the mortality were up to 100% in the group injected with spores (5.0×107 cell/mL) and bacteria(5.0×107 CFU/mL) on dorsal muscle, 80% in the group injected with spores (1.0×108 cell/mL) on dorsal muscle,60% in the group daubed with spores on wound, 20% in the group marinated with culture after ravaged, respectively