Batrachochytrium dendrobatidis causes mortality in various amphibian species including the boreal toad Anaxyrus (Bufo) boreas boreas. The purpose of this study was to determine the physiological effects of this pathogen on experimentally infected boreal toads. Plasma osmolality, sodium, and potassium concentrations were analyzed to evaluate the differences between diseased and non-exposed animals. Infected animals with clinical signs of chytridiomycosis had significantly lower plasma osmolality, sodium, and potassium levels than non-infected animals (p < 0.06). On average, clinically infected animals housed in an aquatic environment had sodium and potassium levels of 60.1 (SE = 9.7) and 2.06 (SE = 0.32) mmol l(-1), respectively. These ion levels were significantly lower than the negative controls (sodium = 115.0 mmol l(-1), potassium = 3.7 mmol l(-1)) and consistent with the clinical signs observed in affected animals. We propose that infection with B. dendrobatidis results in an electrolyte disorder in boreal toads.