ABSTRACT: European catfish Silurus glanis, European eel Anguilla anguilla and rainbow trout Oncorhynchus mykiss were challenged by intramuscular injection of zoospores of Aphanomyces invadans, the oomycete associated with epizootic ulcerative syndrome (EUS). The tropical three-spot gourami Trichogaster trichopterus is known to be highly susceptible and was used as a positive control. European catfish were highly susceptible and rainbow trout had moderate to low susceptibility, whereas eels appeared largely unaffected. Inflammatory host response in European catfish deviated from the effects seen in most other susceptible fish species and was characterised by a more loosely arranged accumulation of macrophages, small numbers of lymphocytes and multinucleated giant cells without occurrence of EUS-characteristic mycotic granulomas. Semi-nested and single round PCR assays were developed for this study to detect A. invadans DNA in clinical samples of experimentally infected fish. The detection limit of the assays equals 1 genomic unit. Specificity was examined by testing the DNA of various oomycetes, other relevant pathogens and commensals as well as host DNA. The single round assay used was fully specific, whereas cross-reaction with the closely related Aphanomyces frigidophilus was observed using the semi-nested assay. Analysis of samples by PCR allowed detection prior to detectable histopathological lesions. Two other published PCR protocols were compared to the PCR protocols presented here.