1. 1. The enzyme catalyzing the CO2-dependent phosphorylation of fluoride by adenosine triphosphate to yield monofluorophosphate, an activity referred to as "fluorokinase," has been isolated in crystalline form from rabbit muscle extracts. The crystalline enzyme has been found to possess considerable pyruvic kinase activity. 2. 2. A number of observations including (a) constancy of pyruvic kinase to "fluorokinase" activity ratio throughout purification and on repeated recrystallization; (b) similar ratio of the two activities for preparations purified by different methods either for pyruvic kinase or for "fluorokinase" activity; (c) requirement of K+ and same broad nucleotide specificity for the two activities (reaction with the nucleoside di- or triphosphates of adenosine, guanosine, uridine, cytidine, and inosine); and (d) inhibition of fluorophosphate formation by phosphoenolpyruvate and by pyruvate, suggest that both the pyruvic kinase and "fluorokinase" reactions are catalyzed by one and the same protein. 3. 3. The possible mechanism of the essentially irreversible "fluorokinase" reaction is discussed.