1. Cephalopod tissues contain a hitherto undescribed phosphagen compound.
2. The rate of acid hydrolysis is comparable with those of creatine and arginine phosphates, but is minimal in N/10 acid, while the retardation produced by the molybdate ion is only about 3-4 times.
3. The finding of a new compound in the Cephalopoda, the Lamellibranchiata being already known to contain arginine phosphate, is possibly of evolutionary significance.