The authors examined the microfilament of fibroblasts in rat calvarial periosteum by transmission electron microscopy after perfusing rats with a mixed solution of Triton X-100 and a low concentration of glutaraldehyde. Fibroblasts in the periosteum were fusiform and had elongated oval nuclei. Microfilaments with diameters of 5-6 nm were noted within the cytoplasm. Although these microfilaments were parallel to the long axis of the cell, stress fibers, which are thick bundles of microfilaments present in cultured fibroblasts, were not observed in these fibroblasts.