We report numerical calculations of the concentration of interstitials in h
ard-sphere crystals. We find that in a three-dimensional fee hard-sphere cr
ystal at the melting point, the concentration of interstitials is 2.7(4) x
10(-8). This is some 3 orders of magnitude lower than the concentration of
vacancies. A simple, analytical estimate yields a value that is in fair agr
eement with the numerical results.