A protozoan infection (Trichodina truttae) was identified in captive Atlantic salmon (Salmo salar) kelts that died in spring of 1988 and 1989. Fish with intense infections showed signs of listlessness, erratic swimming and in appetence. The infection induced excessive mucus secretion, epithelial sloughing and lesions that probably permitted entry of opportunistic bacteria which eventually caused ulcers and death. A seawater bath for 30 min each week for 4 wk effectively controlled the parasite.