Arginase, that regulates metabolism of arginine, is widely distributed in organisms. The two major isoforms, cytosolic Arginase-I, and mitochondrial Arginase-II have been characterized well. However, reports also suggest another mitochondrial membrane-bound arginase which is extracted by washing the mitochondria with KCl. Here, we studied this mitochondrial membrane-bound arginase among vertebrates. Our observations support that arginase activity is predominant in cytosol which is designated as Arginase-I. The mitochondrial membrane-bound Arginase (mbArg) which resembles Arginase-II seems independent of nitrogen excretion pattern because of its presence both in ureogenic and non-ureogenic vertebrates.