Furunculosis was induced in brook trout, Salvelinus fontinalis, by experimental inoculation with Aeromonas salmonicida. Total protein, hemoglobin, sialic acid, fatty acids, triglycerides, cholesterol, inorganic-phosphorus, acid-soluble phosphorus, and lipid-phosphorus decreased in the blood of the infected fish while amino acids, urea, total creatinine, ammonia, and glucose increased. Pyruvic acid, lactic acid, and ascorbic acid values showed no significant change.