Microbiopsies of 38 specimens of human Fallopian tube fimbrial epithelium obtained during hysterosalpingectomy operations were processed for electron microscopic observation. Calcium localization was demonstrated utilizing the pyroantimonate-osminium-tetroxide technique. Calcium deposits were consistently observed as scattered granules in the nuclei, nucleoli, and cell membranes of secretory and ciliary cells regardless of the stage of the menstrual cycle. On the other hand, mitochondrial-bound calcium was observed in the ciliary cells during the preovulatory phase, coinciding with relatively high estrogen levels, and was not found during the peri- and postovulatory phases of the menstrual cycle. It appears that there is cellular calcium which is relatively stable and is probably part of the structural components of the cells. The ciliary cell mitochondrial calcium appears to be more labile and is therefore available for mobilization and utilization in the metabolic processes associated with the periovulatory phase.