Abstract: The study of the physicochemical and catalytic properties of arginase (L-arginine amidohydrolase, EC 3.5.3.1) from the liver of fruit bat (Eidolon helvum, Kerr), a flying mammal, was investigated for the purposes of biochemical and evolutionary comparison with its extensively studied terrestrial mammalian, ureotelic and uricotelic forms. Arginase was isolated from the liver of the fruit bat and purified to apparent homogeneity. The purification procedure involved two ion-exchange chromatography steps on DEAE-Cellulose and CM-Sephadex columns followed by a gel filtration step on Bio-Gel P-l00. The specific activity of the enzyme was estimated to be 36.7 U mg-1. The enzyme exhibited Michaelis-Menten kinetics with a Km of 17 mM for arginine and Vmax of 1.39 μmol mL-1 min-1. The apparent molecular weight estimated on a Sephadex G-200 column was 80,000. Sodium dodecyl sulphate polyacrylamide gel electrophoresis revealed two subunits with molecular weights of 34,000 and 52,000. The enzyme probably exists as two dissimilar subunits or as multimer of the 34,000 dalton protein. Fruit bat liver arginase showed slight thermostability with optimum temperature of 60°C and a pH optimum of 9.0. The enzyme was inhibited from 10-40% by the chloride salts of barium, cobalt, iron, magnesium and nickel at 1.0 and 1.5 mM. From its molecular weight and kinetic parameters, it can be concluded that fruit bat hepatic arginase is similar to the enzyme from ureotelic animals.