We have used for the first time the electron‐microscopy technique to reveal the pure ferroelastic phase domain structure of lead phosphate, Pb3(PO4)2. The domain configuration confirms previous identifications of the symmetry elements lost at the transition. The domains are found separated by thin walls whose thicknesses are less than 50 A˚. This and other recent studies suggest narrower walls in ferroelectrics and in ferroelectric ferroelastics than were envisaged from earlier theoretical models. The very high values reported by different authors are believed to be due to the resolution limitations of the experimental setup used. Fruitful consequences are expected from a thorough understanding of the stresses and lattice distortions associated with the ferroelastic domain structure.