The press juice from mature seedling wheat leaves possesses a high thermo-labile activity in the inversion of sucrose. It is concluded that an invertase is present. The active material is not associated with the coarse particles of the protoplasm but is soluble in the sap and in that condition is very stable in the cold. It is, however, sensitive to drying and precipitation. Some kinetic characteristics of this enzyme resemble those of invertases from other sources. But the Michaelis constant is small by comparison with that of yeast invertase and therefore a relatively high affinity of the leaf enzyme for its substrate is indicated.