Winter kill, sometimes referred to as winter Saprolegniasis, annually produces significant mortalities in the farm-raised channel catfish industry of the southeastern United States. Several theories have been put forward concerning the dehydration, immunosuppression and fungal infections seen during post-mortem examinations of affected fish. We reviewed cases from winter 1994-5. The principal lesions are in the skin including ulceration with subacute to chronic inflammation of variable depth (occasionally into the musculature). Other changes identified are hepatocellular atrophy, immature hematopoietic hyperplasia in anterior and posterior kidney, and splenic congestion. Fungi, although frequently suspected, are not seen in all sections of affected skin.