The fine localization of mineral matter in spores of Bacillus megaterium and Bacillus cereus was studied by the technique of microincineration adapted for use with the electron microscope. The specimens, which included intact and thin-sectioned spores as well as shed spore coats, were burned either in the conventional way at high temperature or by a new technique using electrically excited oxygen at nearly room temperature. The ash residues were examined by bright field, dark field, and diffraction in the electron microscope and also with the phase contrast microscope. In some cases, the specimen was previewed in both microscopes before incineration. The results do not support a previous report that the mineral elements of the spore are confined to a peripheral layer, but rather indicate that the spore core as well as the coat are mineral-rich. The cortex may be deficient in minerals, but the possibility of artifact prevents a clear decision on this point. Incinerated B. megaterium spores show a highly ordered fine structure displaying 100 A periodicity in the ash of the middle layer of the coat. The nature of this structure is discussed, as is the technique which demonstrated it. The fine definition of the ash patterns, particularly those obtained with the low-temperature, excited-oxygen technique, suggests that microincineration may be generally useful in the study of fine structure.