Mass death of farmed brook trout Salvelinus fontinalis reared in raceway culture on a commercial fish farm was recorded in May. The condition was caused by natural infection with external signs of Flavobacterium columnare. Haematological analyses were carried out from peripheral blood from 10 fish from each group. Anaemia was characterized by a reduced erythrocyte count and lower haemoglobin levels; the values of mean corpuscular volume and mean corpuscular haemoglobin were higher and the values of mean corpuscular haemoglobin concentration were decreased. Clinical chemistry analyses in blood plasma indicated decreased levels of total protein, glucose and total calcium and an increase in the urea level. Among the four enzymes analysed, a catalytic activity reaching multiples of the normal level was found in alanine aminotransferase, aspartate aminotransferase and lactate dehydrogenase. The catalytic concentration of alkaline phosphatase was decreased. Electrophoretic analyses of the plasma protein indicated a reduced level of albumin. The moribund fish had a higher hepatosomatic index and their condition coefficient after Fulton and after Clark was also higher.