The aims of this project were to: 1) provide an understanding of the general biology and ecology of the fungal pathogen of epizootic ulcerative syndrome (EUS) in fish and resolve questions over its relationship with strains from other similar diseases, and (2) to examine immune mechanisms of fish against infection by the EUS fungal pathogen. The study showed that the newly named Aphanomyces invaderis is the only species of a number of isolates from EUS-affected areas in Thailand that is capable of invading snakehead fish muscle tissue and reproducing EUS lesions. Its characterization and differentiation from saprophytic isolates is described. A. invaderis was shown to be indistinguishable from pathogenic Aphanomyces isolates from 2 other fish diseases: Japanese mycotic granulomatosis and Australian red spot disease. Isolates from outbreaks of ulcerative mycosis of American menhaden were shown to be distinct from A. invaderis. Snakeheads (Channa striata) and trout (Oncorhynchus mykiss) were shown to produce non-specific antibodies in response to A. invaderis infection. The cellular immune response was also studied. Other characteristics of A. invaderis, including its susceptibility to chemical treatment are also reported.