Magnesium oxide crystals have been examined in an ultramicroscope and the arrangements of precipitate particles in the material are illustrated. Unusual dislocation configurations are revealed by decoration in certain crystals.It has been suggested in the literature that most precipitate particles in magnesia are of zirconium oxide. The present observations of particle density indicate that it is unlikely that this is completely true at least for heat‐treated samples, even if the particles are finer than has formerly been assumed.