The ultrastructure of the inner myocardium of aruana, an osteoglossid water breather, and Arapaima, an air-breathing Amazon relative, was compared. The aruana heart was laden with glycogen granules while Arapaima heart was fat loaded. Associated with air breathing in Arapaima, the ultrastructure of the inner myocardium displayed abundant mitochondria, clearly differentiated into myofibrillar and peripheral populations. As many of the mitochondrial characteristics of Arapaima resembled those in the mammalian heart, it was postulated that the inner myofibrillar mitochondria are probably specialized for oxidative metabolism as is the case also in the mammalian heart, while subsarcolemmal mitochondria were specialized for the exchange of materials with the blood. In contrast with the heart of the air breather, in aruana the inner myocardium contained two cell types. Type I cells like those in Arapaima myocardium were specialized for aerobic metabolism, displaying abundant mitochondria mostly myofibrillar in location and ample glycogen granules as a potential substrate source. Type II cells by comparison contained fewer mitochondria, but were rich in glycogen granules, and appeared specialized for anaerobic metabolism.