The development of fungus (Saprolegnia sp.) infection in the freshwater teleost Catostomus commersoni Lacepede may be facilitated by environmental water temperatures exceeding 10 C. After prophylactic treatment with a 1% Malachite green solution, fungus growth could be observed either at water temperatures exceeding 13 C or after intravenous administration of 17-hydroxysteroids (cortisol or cortisone), estradiol, progesterone or TSH, independent of the water temperature. Corticosterone, testosterone or ACTH did not have this facilitating effect. The problem of the lethal fungus infection in anadromous Salmonidae is discussed. It is suggested that high plasma levels of estrogens as well as 17-hydroxysteroids may play a decisive role in the fungus infection and subsequent breakdown of osmoregulatory mechanisms leading to the death of these fish on their spawning grounds.