The distribution of adenosine triphosphatase (ATPase) activity in the phloem of petioles and minor veins of Cucurbita maxima has been studied using a lead phosphate precipitation procedure. ATPase activity was localized in sieve elements, companion cells and parenchyma cells. Activity was found at the cell surfaces, associated with the dispersed P-protein of mature sieve elements, in mitochondria, sieve-element reticulum, and at specific regions of the cell walls. It is suggested that the ATPase at the phloem cell surfaces may function in intercellular transport of assimilates or ions, and that the ATPase activity associated with the P-protein may function in the translocation process or in callose deposition.