Integrin-mediated interaction with the extracellular matrix plays a critical role in the function of osteoclasts, the bone-resorbing cells. This study examines the role of p130Cas (Crk-associated substrate (Cas)) in actin organization in osteoclasts. Multinucleated osteoclast-like cells (OCLs) were obtained in a co-culture of murine bone marrow cells and primary osteoblasts. After plating on culture dishes, OCLs formed a ringlike structure consisting of F-actin dots at cell periphery (actin ring). The percentage of OCLs with actin rings and its diameter increased with time and cell spreading. Tyrosine phosphorylation of a protein (p130) increased with actin ring formation. Treatment with cytochalasin D disrupted actin rings and reduced tyrosine phosphorylation of p130. Using specific antibodies, p130 was identified as Cas. By immunocytochemistry, Cas was localized to the peripheral regions of OCLs and its distribution overlapped that of F-actin. In OCLs derived from Src(-/-) mice, in which osteoclast activity is severely compromised, tyrosine phosphorylation of Cas was markedly reduced. Moreover, Cas was diffusely distributed in the cytoplasm and actin ring formation is not observed. These findings suggest that Src-dependent tyrosine phosphorylation of Cas is involved in the adhesion-induced actin organization associated with osteoclast activation.