Fungal mycelium extracts were subjected to gel electrophoresis and Western blot analysis with various lectins and polyclonal antisera. These techniques succeeded in identifying bands specific to pathogenic Aphanomyces strains from fish affected by epizootic ulcerative syndrome, redspot disease and mycotic granulomatosis when compared to various other Oomycete fungi. It is concluded that these isolates are conspecific under the name Aphanomyces invadans [previously described by Willoughby et al. 1995 (J Fish Dis 18:273-275) as A. invaderis] and the various protein and carbohydrate bands identified may provide useful species-specific taxonomic markers. Besides the distinctive band profiles for A. invadans on gels subjected to various stains, lectins revealed a specific 45 kDa component, and polyclonal antisera highlighted an immunogenic 10 kDa band. Peroxidase and fluorescein conjugated antisera provided an effective diagnostic tool for identifying hyphae in infected fish tissue.