Electrophoretic studies have been made of the protein composition and isozymic pattern of the creatine kinase from muscles of the cod. It was shown that this enzyme constitutes up to 40% of total water-soluble proteins of muscle tissue. Isolation and purification procedures were suggested for the creatine kinase from cod muscles which allow to obtain the enzyme with the specific activity 250-350 IU/mg. Comparative enzymic analysis of creatine kinases from muscle tissue of the cod, lake frog, and rabbit was also made. Studies were carried out on temperature dependence of the reaction, kinetic constants at temperatures 5 and 30 degrees C were determined together with other physicochemical properties of the enzymes. The revealed species specific differences are discussed in relation to the structure and function of the enzymes in lower vertebrates in vivo.