The spatial fluctuations of lattice strain in regularly grown and in rapidly cooled Czochralski-grown silicon (CZ Si) wafers were measured by means of plane-wave X-ray topography using highly collimated X-rays with an angular divergence of less than 0.01”. By comparison with the spatial fluctuations of the interstitial oxygen (O_i) concentration measured by means of micro-Fourier transform infrared spectroscopy, we found that the lattice strain of the rapidly cooled wafer was smaller than the strain attributed solely to O_i atoms. This result gave the first clear evidence for the existence of vacancy-type defects in as-grown, rapidly cooled CZ Si crystals.