A monoclonal antibody (MAb), designated 3gJC9, was raised against a protein antigen of Aphanomyces invadans, the oomycete pathogen that causes epizootic ulcerative syndrome (EUS). The antigen was expressed on the surface of hyphae and secreted extracellularly. MAb 3gJC9 did not cross-react with other oomycete or fungal pathogens of fish, although it did react to the crayfish plague pathogen A. astaci. The MAb was used for immunofluorescent staining on histological sections of fish infected with EUS, and was found to be more sensitive than conventional staining methods for detecting A. invadans. It thus has utility in confirming the case definition of EUS. It also revealed very small filamentous structures, the significance of which is unclear, but they may represent an early stage of infection, thus allowing earlier detection of the disease, since they are not detected using conventional staining methods.