Abstract The influence of an acute drop in water temperature on channel catfish ( Ictalurus punctatus ) mucous cell distribution in the epidermis was examined. Channel catfish were subjected to a water temperature drop from 22 to 10° C in 24 h and sampled for histological examination of the epidermis at regular intervals. Three days after the drop in water temperature, there was a marked decrease in the number of mucous cells in the outermost (superficial) portion of the epidermis. The number of mucous cells returned to normal levels after fish were maintained at 10° C for 6 days. If catfish were subjected to the temperature drop and also challenged with Saprolegnia , mucous cells in the superficial epidermis did not recover to normal levels but continued to decline from day 3 until the fish subsequently died from saprolegniasis. These results may explain the dry skin appearance characteristic of catfish exhibiting winter saprolegniasis. Furthermore, since Saprolegnia cysts are normally shed in sloughed mucus, low temperature-induced mucus loss may explain how Saprolegnia cysts attach and infect catfish skin to result in disease. These results also reinforce the notion that an intact external mucus layer is the first line of defence against infectious disease in fish.