SUMMARYExperimental vesicular calculi were produced in albino rats by feeding the animals on a diet deficient in vitamin A. The distribution of mineral salts in these calculi was studied by microradiography. The nature of the mineral salts was determined by X‐ray diffraction methods. Calculi with a layered deposition of the mineral salts gave an X‐ray diffraction pattern characteristic of hydroxylapatite. Such layered calculi are found both among experimental and spontaneous calculi. Their genesis is briefly discus