Transmission electron micrographs of individual mineralized collagen fibrils show that hydroxyapatite crystals are located mainly within the fibrils at the level of the gap regions. The plate-shaped crystals are observed to be more or less uniformly stacked across the fibril diameter. We therefore suggest that the crystals are primarily located in 'grooves' created by contiguous adjacent gaps. The proposal is consistent with the observed crystal distribution in the fibril and with their average widths, which are almost 10-times greater than an individual gap diameter.