Summary.1. The creatine of vertebrate muscle is mainly present in the form of a compound with phosphoric acid.2. This labile compound is intimately associated with muscular contraction. It is broken down in activity and reconstituted during rest, and is a more immediate energy source than is glycolysis. It is also broken down under conditions which are unfavourable to the organism as a whole.3. Similar considerations appear to hold in the case of the electrical organs of certain fishes, and may possibly hold for ciliary motion also.4. The creatine of the vertebrates is replaced, in many if not in all invertebrates, by arginine, which also forms a labile compound with phosphoric acid, and this is of a physiological significance exactly parallel to that of creatine phosphoric acid.5. The preparation, properties, and physiological behaviour of these compounds are described and discussed, while the methods in common use for their estimation are also described.6. Guanidine derivatives other than creatine and arginine may also behave in the same way, giving phosphagenic compounds of similar functional importance. One such compound appears to be present in cephalopod muscle.7. Creatine phosphate is practically confined to the vertebrates, whereas arginine phosphate is never found in members of that phylum. Both compounds have been found together in an echinoid and in an enteropneust. The Echinoderm‐Enteropneust theory of vertebrate affinity, previously postulated on purely morphological grounds, seems thus to find new support on biochemical grounds.8. The Cephalopoda appear to contain a phosphagen whose base is not arginine, whereas arginine phosphate appears to be present in the Lamellibranchiata. This is possibly of evolutionary significance, since the Lamellibranchiata are believed to have branched off from the main line of evolution at a date considerably earlier than that of the divergence of the other Mollusca.9. Protozoa appear not to contain phosphagen, but with a possible exception in the case of the Coelenterata, the Metazoa all make use of some phosphagen compound.The author wishes to express his gratitude to Dr Joseph Needham and Mrs Needham for their continual help and advice, and to the Department of Scientific and Industrial Research for a grant, during the tenure of which this article was writte