1.Phospholombricine and phosphoarginine are the two naturally occurring phosphagens in the facultative anaerobeTubifex(Table 1). During experimental anaerobiosis concentrations of both phosphagens decreased within the initial 6–12 h (Fig. 1).2.Lombricine has been isolated from the worms with a yield of 7.2 μmoles per g d.wt. and purified to homogeneity. Phospholombricine was obtained by enzymatic transphosphorylation ofd-lombricine. The compound was also isolated from the worms by a specific procedure.3.The 1,473-fold purified enzyme lombricine kinase has a specific activity of 30 U/mg of protein at 25°C (Table 3) and a turnover number of 2,100 min−1. The molecular weight was estimated to be 69,000.4.Kinetic and regulatory properties of both phosphagen kinases and of adenylate kinase (myokinase) are reported (Fig. 2, Table 4).5.In living worms the ATP/ADP ratio, or the energy charge value, probably determines whether the phosphagens are synthesized or uti