Furunculosis was induced in brook trout,Salvelinus fontinalis, by experimental inoculation withAeromonas salmonidda.The levels of blood enzymes were determined and the most significant modifications were observed for aldolase, creatine phosphokinase and ornithine carbamyl transferase. Alkaline phosphatase, glutamic‐pyruvic transaminase and glutamic‐oxalacetic transaminase showed minor modification. There was no significant change of activity for acid phosphatase, cholinesterase, lactic dehydrogenase, phosphohexose iso‐merase, isocitric dehydrogenase and aminopept