Many diseases of aquatic animals are caused by organisms that are part of the normal biota of their surrounding environment. Fungi and fungal-like organ-isms (straminipilous organisms) occur in most waters. They are either saprobes that colonize decaying organic matter, or parasites, which attack a great variety of aquatic organisms leading to disease outbreaks. Among the parasitic species, many are known to be important causative agents of aquatic animal diseases. They are generally opportunistic invaders, but once established, are often fatal and difficult to treat. Thus, fungi and straminipilous organisms may be prob-lematic pathogens under stressful conditions in an aquaculture system.