The clinical signs and histopathological disturbances produced by Saprolegnia sp. are described in Chondrostomos polylepis and Rutilus albugineus, under environmental stress. The main alterations observed were loss of epithelium leading to ulceration and blood disorders, consisting of blood congestion and occasional hemorrhages. Infected fish developed focal lesions with the fungus invading the stratum spongiosum of the dermis before extending laterally over the epidermis. The onset of disease was brought about by a combination of a rapid fall in water temperature, which probably induced an effect of immunosuppression, and the maintenance of low water temperature, which favored the proliferation of the fungus.