Epizootic ulcerative syndrome (EUS), caused by is an invasive disease of both wild and farmed fish in freshwater and estuarine environments. After it was first described in Japan in 1971, the disease spread widely across Asia and to some regions of Australia, North America, and Africa. In Asia and Africa, the spread of the disease has substantially affected the livelihoods of fish farmers and fishermen. No reports on the presence of the disease have been made in China. In 2012, a similar EUS of broke out in Guangdong Province, China. The ulcerative clinical signs were observed on the dorsal, ventral, and caudal dermal and muscular layer, and were especially prominent on the nape, opercular, and truncal regions. To diagnose the with the suspected EUS, isolation and identification of the mold pathogen were conducted, histologic observations were made of ulcers, and morphology observations were made. The technologies, which were used for the isolation, purification, pathological histology, squashing of tissue, scanning electron microscope technology, and HE and Grocott hexamine silver staining were applied for the identification of the pathogenic morphology, molecular biology, and histopathological observation of the suspected epizootic ulcerative syndrome. The typical filamentous hyphae were isolated from the ulcerated tissue and morphology of the and the original generation of spore were observed under optic microscopy. The hyphae were nonseptate with irregular walls and occasional branches and approximately 10-20 μm in diameter. ITS sequence amplification analysis showed that it was 100% homologous with . The histopathologic changes of the infected fish were observed under optic microscopy. A highly invasive fungus was associated with the EUS in . By squashing the tissue of the ulcerative dermal and muscular layers, large numbers of hypha were observed. A large number of fine filamentous fungi were observed in the muscle tissue by SEM. At necropsy, ulceration and muscle atrophy were noted. The histological sections of skin from multiple ulcerative areas had exuberant hyphae invading through the ulcerated skin. The fungus invaded and proliferated tissues away from the site of dermal ulcers and even penetrated into the skeletal muscle. In more severely affected areas, granulomatous inflammation was also present, and was characterized by numerous macrophages, few lymphocytes, and scant cellular debris. The surrounding muscle fibers were fragmented, vacuolated, and disrupted with loss of cross striations and separation of the fibers. Using Grocott hexamine silver stain, the section of the ulcerative areas more clearly showed the hyphae. The disease was diagnosed as EUS.