Defect selectiveA/Bchemical etching, low‐temperature scanning cathodoluminescence, and transmission electron microscopy have been used to study the microstructure of undoped, semi‐insulating liquid‐encapsulated Czochralski GaAs wafers. It is shown for the first time that a distribution of microvoids is present in the bulk material at a number density of at least 1010cm−3. These microvoids, which are present in the centers of the dislocation cell structures observed in the GaAs, may result from the post‐growth heat treatment of ingots which is used to improve the material homogeneity. A possible explanation for the formation of these microvoids is given.