Based on their ultrastructural features, osteocytes have been classified into three phases: formative, resorptive, and degenerative. However, the mechanism of the morphological changes from formative osteocyte to resorptive osteocyte is still unclear. Therefore, we labeled bone matrix with calcein and examined chronologically the histochemical changes in osteocytes from two distinct areas in the mandible, focusing on the relationship between osteocytes and osteoclastic bone resorption. Tartrate-resistant acid phosphatase (TRAP) activity was detected only in osteocytes located near sites of osteoclastic bone resorption. Osteocytes far from osteoclasts lacked TRAP activity, even those that had been enclosed in bone matrix as long as the osteocytes that did show TRAP activity. Furthermore, glycogen granules evidenced by periodic acid-Schiff and the periodic acidthiocarbohydrazide-silver protein staining in osteocytes disappeared when the cell acquired TRAP activity. These findings suggest that the acquisition of TRAP activity in an osteocyte is closely synchronized with bone resorption. Disappearance of glycogen in osteocytes in the same area may also be related to the acquisition of TRAP activity or bone resorbing activity. The osteocytes characterized by glycogen accumulation must be in the phase that follows the formative phase, in which osteocytes display neither formation nor resorption activity.