A diverse array of phosphagen kinases [arginine kinase (AK), lombricine kinase (LK)], glycocyamine kinase (GK), taurocyamine kinase (TK), and creatine kinase (CK) is found in the animal kingdom (see ref. 1 for a review). These reactions appear to function in the temporal buffering of ATP in muscles during energy deficits such as might occur during burst contraction or anoxia (2, 3). In many vertebrate tissues, a distinct mitochondrial isoenzyme of CK is present, and it may play a special role in the intracellular transport of high energy phosphate (4). In this study, we investigated whether mitochondrial activities of phosphagen kinases are present in invertebrate muscles. Our results show that AK is present in mitochondria from a crustacean. However, phosphagen kinases are lacking in mitochondria from insect flight muscles, molluscan cardiac and smooth muscle, and polychaete and oligochaete body wall musculature. It appears that mitochondrial activities of phosphagen kinases are not widely distributed in the invertebrates. These data, in conjunction with previous studies on the physico-chemical nature of the interaction of phosphagen kinases with mitochondria (5, 6), suggest that mitochondrial compartmentation of phosphagen kinases may have evolved independently in two major animal groups.