We investigated the feasibility of monitoring the health of Atlantic salmon to facilitate development of an early warning system for disease. To do this, we recorded changes in the swimming behavior and blood composition in the salmon after challenge with challenge experiments were the same batch from the same farm. Healthy fish and fish infected with colony forming units (CFU)/mL per fish (suspended in saline)control fish were injected with isotonic sodium chloride 100 µL per fish)The swimming ability and tail beat frequency measurements of the healthy and sick fish a significant decrease in the critical swimming speedPThere was also a significant change in the slopes of the correlations between the tail beat frequency and the swimming speed0.05, comparisons of the leukocyte system were made between groups at each sampling point and among treatment groups for time after challengethe total number of leukocyteschallenged groups compared with the control groups the number of leukocytes and granulocytes significantly reduced by 2.8% and 43.9% 0.05and the number of lymphocytes and monocytes significantly increased by 63.3% and 23.9% 0.05there were significant correlations between numbers of blood cells and the sampling point in the challenged fish0.05). The results of the present study highlight the feasibility of monitoring fish health through use of the following convenient parameters: swimming performancecritand blood analysis.lay the foundations for an early warning system for fish disease.