1. 1. An enzyme which hydrolyzes inorganic pyrophosphate to orthophosphate has been partially purified from extracts of firefly lanterns. 2. 2. Magnesium ion is essential for enzyme activity; Mn, Zn, Ca, and Be ions are completely inactive. Magnesium ion, at a concentration of 0.00045 M gives 50% of maximum activity. 3. 3. The maximum rate of substrate hydrolysis is observed at pH 8.3 and at a substrate concentration of 0.0004 M. Higher concentrations of the substrate inhibit the enzyme. 4. 4. The optimum temperature was 35 °C., and the experimental energy of activation of the reaction was approximately 37,000 calories. 5. 5. No inorganic phosphate was liberated when the enzyme was incubated with the following substrates: adenosine triphosphate, adenosine diphosphate, thiamine pyrophosphate, coenzyme I, sodium triphosphate, glycerophosphate, hexose diphosphate, sodium meta- and hexametaphosphates. 6. 6. The enzyme is inhibited by Cu, Ca, Be, and Mn ions and by NaF. Calcium competes with the Mg ion; 50% inhibition is observed at a Ca Mg ion ratio of 0.1. Manganese inhibition depends upon the substrate concentration. Iodoacetate and alloxan at 0.01 M concentration have no effect on enzyme activity in contrast to other pyrophosphatases. No activation with cysteine has been observed. 7. 7. The enzyme concentration in the firefly lanterns is over ten times greater than that found in the remainder of the body.