1. The Michaelis constant (Kin) of the trout pancreatic lipase is 1.3 x 10-6M tributyrin, calculated by the interface Concentration of the emulsion. This value is lower than that of porcine pancreatic lipase. 2. The lipase hydrolyses tributyrin in a Ca 2÷ free medium. Conversely, Ca 2+ is essential for the lipolysis of triolein. The cation might be an effector of the reaction, but it seems to remove the inhibition of the enzyme by its product. 3. The curves of the lipase activity according to bile salt concentration seem to suggest the existence of a colipase, that we have not evidenced yet by direct procedures. 4. The apparent molecular weight of the lipase seems to be lower in the trout than in the species studied so far.