X‐ray diffraction patterns of calcium samples of different purities have shown that 99.9+% Ca exists in only two allotropic forms: face‐centered cubic to 464°C and body‐centered cubic from 464°C to the melting point. These results for the high temperature allotrope differ from the widely accepted structure; handbooks and tables currently list the high temperature allotrope as hexagonal closest‐packed. It has been further shown that a previously reported intermediate allotrope of complex structure is due to contamination. The temperature dependence of the electrical resistivity of the 99.9+% Ca was found to be linear for both the face‐centered cubic allotrope and for the body‐centered cubic allotrope. Calcium was observed to be self‐annealing at room temperature.