Aphanomyces invadans, also referred to as A. invaderis or A. piscicida, is the aetiological agent of epizootic ulcerative syndrome (EUS). Immune responses of rainbow trout (Oncorhynchus mykiss) against this fungus were examined, including both antibody and macrophage responses. The fish antibodies to the fungus, assessed by an enzyme-linked immunosorbent assay (ELISA) were generally non-specific, but sera from fish vaccinated with A. invadans reacted with bands at 55 and 40 kDa by Western blot analysis. Macrophage activity was examined in vitro by culturing rainbow trout head kidney macrophages with fungal spores. The macrophages were able to phagocytose spores in vitro and, in some cases, hyphae were observed growing from germinated spores within the macrophage. When low concentrations of spores (1??102 and 1??103 spore ml-1) were cultured with macrophage monolayers, no germination of spores or growth of hyphae was observed. At concentrations of 1??104 spore ml-1 or higher, macrophages were unable to kill all the spores and fungal mycelium eventually overgrew the culture. Transmission electron microscopy (TEM) was used to examine the ultrastructural interaction between macrophages and the fungus. The possible role of rainbow trout as a model for A. invadans infection in salmonids is also discussed.