Fecal stones or fecal masses in the appendix as well as the normal feces were investigated by X-ray analysis. 1. The crystal substance contained in the fecal stone and the fecal mass in the appendix was considered of apatite, starch and a substance with unknown diffracting shade, while the crystal substance in the normal feces was only consisted of starch. Thus, the most important component of the fecal stone is the crystal of apatite, which emerges with a process of inflammatory change. A layer construction of the calcium and the feces was crystallographically obseved in the fecal stone. In severe appendicitis, the macroscopic finding of the fecal stone considerably corresponded with its crystallographic classification, but not in the mild one. Several fecal stones or masses in the same appendix represented almost the same crystallographic findings.