The distribution and orientation of biological apatite crystals in calcified turkey leg tendons were studied by selected-area dark field electron microscopy. This imaging technique enables the direct visualization of apatite and the specific determination of the crystallographic axes (a, b-axes or c-axis) within calcified collagen fibrils. This study shows that at early stages of mineralization, rod-shaped apatite crystals (5–20 nm in length) were localized and dispersed within gap zones bordering both the collagen molecule C- and N-terminal regions. At later stages of mineral deposition the crystals were more extensive, occupying greater areas of the gap zone and, in addition, apatite crystals were found to occur in the overlap zones. The orientation of apatite crystals was observed to be an alternating and interlocking distribution of a, b-axes and c-axis along the axial period of collagen fibrils. This distribution is interpreted as representing a continuous rotation of apatite axial orientation along the collagen period.