Alveolar bone destruction is a characteristic of periodontal disease. Treponema denticola are found in significantly increased numbers in the sites affected with periodontal disease. In order to clarify the role of T. denticola in destruction of alveolar bone in periodontal disease, this study was undertaken to determine the effect of sonicated extract of T. denticola on osteoclast differentiation in co-culture system of mouse bone marrow cells and calvaria cells. The ability of osteoclast formation was estimated by counting the number of tartrate resistant acid phosphatase(TRAP) positive cells. Sonicated extract of this bacteria stimulated osteoclast formation in a dose dependent manner(p<0.05). Indomathacin, an inhibitor of prostaglandin synthesis, decreased osteoclast formation induced by sonicated extract of this bacteria(p<0.05). Extract-induced osteoclast formation was decreased, when sonicated extract of bacteria was heated(p<0.05). These findings suggest that T. denticola induces osteoclast differentiation, and protein component of this bacteria and PGE₂ may play an important role in this process.