Vacuolar membrane H+-translocating pyrophosphatase (H+-PPase) was purified from pumpkin seedlings. Its enzymatic properties including molecular size of constituting polypeptide (75 kDa) were very similar to those of mung bean H+-PPase [(1989) J. Biol. Chem. 264, 20068–20073]. The native, functional molecular size of the pumpkin H+-PPase was estimated to be 135–139 kDa from gel permeation HPLC of the purified enzyme in the presence of detergent and from radiation inactivation of the enzyme in vacuolar membranes. It is concluded that native, functional pumpkin H+-PPase, and also probably H+-PPases from other plants, is a dimer of 75 kDa subunits.