The phosphagen kinases are a family of enzymes that catalyze the reversible phosphorylation of specific phosphagens using ATP as the phosphate donor. The evolutionary relationships between the enzymes in this family have been studied for over 30 years and yet aspects of the relationships remain unclear. Here, arginine kinase from Strongylocentrotus purpuratus eggs was purified to homogeneity and analyzed for physical and kinetic characteristics as well as its sequence homology with other phosphagen kinases. The results indicate that dimeric arginine kinase from S. purpuratus eggs evolved from a dimeric creatine kinase after dimeric creatine kinase evolved from a monomeric arginine kinase. The molecular weight/subunit composition and sequence of dimeric arginine kinase from S. purpuratus eggs is more comparable with dimeric creatine kinases than monomeric arginine kinases. However, the kinetic characteristics of dimeric arginine kinase from S. purpuratus eggs, including the absence of substrate cooperativity, are more comparable with other arginine kinases than creatine kinases. These results indicate a unique evolution for the dimeric arginine kinase, as well as the importance sequence composition can have on the three dimensional structure of an enzyme and its kinetic characteristics.