Solubilization of bone mineral by osteoclasts depends on the formation of an acidic extracellular compartment through the action of a V-proton pump that has not yet been characterized at the molecular level 1, 2, 3. We previously cloned a gene (Atp6i, for V-proton pump, H+ transporting (vacuolar proton pump) member I) encoding a putative osteoclast-specific proton pump subunit, termed OC-116kD (ref. 4). Here we show that targeted disruption of Atp6i in mice results in severe osteopetrosis. Atp6i–/–osteoclast-like cells (OCLs) lose the function of extracellular acidification, but retain intracellular lysosomal proton pump activity. The pH in Atp6i–/–liver lysosomes and proton transport in microsomes of Atp6i–/–kidney are identical to that in wild-type mice. Atp6i–/–mice exhibit a normal acid-base balance in blood and urine. Our results demonstrate that Atp6i is unique and necessary for osteoclast-mediated extracellular acidification.