A constitutive arginase has been purified about 260-fold from Evernia prunastri thalli incubated for 16 hr on 40 /iM cycloheximide. This enzyme has a molecular mass of 36.5 kDa and only one active site, which implies an absence of cooperativity when substrate binds to the enzyme. When evernic acid binds to arginase, the enzyme polymerizes and the number of interaction sites with the substrate increases from 1 to 3-4. This is interpreted as a mechanism of positive cooperativity in the binding of ligand to arginase. Evernia prunastri, constitutive arginase, activation, evernic acid, polymerization