Distributions of actin filaments in osteoclasts on chicken medullary bone were examined during the egg-laying cycle. When an egg was in the magnum or isthmus of the oviduct at 6 h after oviposition, the osteoclasts lacked ruffled borders and attached to medullary bone surface via clear zones. Actin filaments were localized as amorphous bands in the bone site of the cytoplasm, corresponding to clear zones in the osteoclasts. When an egg was in the shell gland of the oviduct at 15 h after oviposition, the osteoclasts had long and slender, well-developed ruffled borders. Actin filaments were localized as lines perpendicular to the bone surface at the bone site of the cytoplasm. In medullary bone osteoclasts, actin filaments changed during the egg-laying cycle. The relationship between the actin filaments and the ultrastructure of the osteoclasts are discussed in this report.