Of the six phosphagen kinases found in animals, the primary structure is known only for creatine kinase. Here we report three cDNA-derived or chemically determined amino acid sequences of two kinds of phosphagen kinases: a glycocyamine kinase from the polychaete Neanthes diversicolor (Annelida) and arginine kinase from the abalone Nordotis madaka (Mollusca) and the shrimp Penaeus japonicus (Arthropoda). Like vertebrate creatine kinases are monomers. These enzymes consist of 350 to 390 amino acid residues, and have a calculated molecular mass of 39,900 to 44,500 Da. Neanthes glycocyamine kinase shows 50 to 58% sequence similarity with vertebrate and invertebrate creatine kinases, having the greatest similarity (57 to 58%) with vertebrate mitochondrial creatine kinase isoform. It shows lower, but significant similarity (37 to 39%) with invertebrate arginine kinases. The sequence similarity between Nordotis and Penaeus arginine kinases is 51%. A phylogenetic tree constructed from 14 amino acid sequences of phosphagen kinases showed that they can be separated into three major clusters corresponding to creatine kinase, glycocyamine kinase and arginine kinase. The cluster of glycocyamine kinase is apparently closer to that of creatine kinase than arginine kinase. The cluster of creatine kinase is composed of several subclusters, each corresponding to three vertebrate isoforms and the invertebrate enzyme.