An epidemic of trichodinosis associated with severe epidermal hyperplasia occurred in adult largemouth bass (Micropterus salmoides) from the Chowan River drainage, North Carolina (USA) in late winter to early spring 2002. Initial reports by anglers of fish with a "jelly-like slime coat" on the skin prompted an electrofishing survey in which about 10% of sampled largemouth bass had a very thick, bluish-white "mucoid layer" on the body and fins. Moderate to heavy infestations of the ciliate Trichodina were detected in wet mounts of skin from five of five fish having the mucoid layer; these fish also had significant gill infestations. An additional two fish with only mild reddening and four asymptomatic fish (no skin lesions) had mild skin infestations but no gill infestations. Two asymptomatic fish had no skin parasites. Four fish with the mucoid layer were necropsied and had extremely severe epidermal hyperplasia on the body and fins. The hyperplasic epidermis had relatively few mucus cells and typically was about 5-10 times thicker than healthy epidermis. The upper four fifths of the epidermis consisted of finely vacuolated, highly flattened, somewhat disorganized epithelial cells. No other significant clinical or histopathologic abnormalities were detected. No systemic infection by pathogenic bacteria was noted. The environmental cause of the epidemic is uncertain but the lesions suggest that some chronic stressor was involved.