Aquaporins (water channels) selectively enhance water permeability of membranes. Since osteoclast differentiation includes a dramatic increase in cell volume, we hypothesize that aquaporin(s) is/are critical for the formation of the multinucleated osteoclast from its mononuclear precursor. Our studies employ two cell models, bone marrow macrophages (BMMs) and the murine macrophage-like cell line, RAW264.7, as osteoclast precursors. Receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL) and macrophage-colony-stimulating factor or RANKL alone were used to induce osteoclast differentiation in BMMs or RAW264.7 cells, respectively. We first used qualitative reverse transcription (RT)-PCR to examine which of the aquaporins are expressed in osteoclasts and in their precursor cells. Out of the 10 aquaporins examined, only aquaporin 9 (AQP9) was expressed in osteoclast-lineage cells. AQP9 has unique aqueous pore properties mediating the passage of a wide variety of non-charged solutes in addition to water. Western analyses using specific antibodies revealed a higher AQP9 level in RANKL-treated than in untreated cells. Quantitative real-time RT-PCR analyses also demonstrated higher AQP9 mRNA levels in RANKL-treated cells. Finally, we examined the effect of phloretin, an AQP9 inhibitor, on RANKL-induced osteoclast differentiation. Cells were incubated with RANKL for 5 days, and phloretin was added for the last 2 days, when most fusion occurs. A dramatic reduction in osteoclast size and in the number of nuclei per osteoclast was observed in cultures containing phloretin. The inhibitor did not have a significant effect on the number and size of mononuclear phagocytes in cultures not treated with RANKL. Our results suggest a role for AQP9 in osteoclast differentiation, specifically in the fusion process.