The microstructure of a perovskite, La2MnGaO6, was studied by Transmission Electron Microscopy (TEM). An idiomorphic grain exhibited a domain structure at ambient temperature, and each domain possessed a common b-axis and axial relation, [100]//[001]. When the grain was heated by a convergent electron beam, the domains vanished after the orthorhombic to rhombohedral phase transformation taking place at 423 K. The domains reappeared when cooled, although domain boundaries moved and the domains rotated by 90degrees with respect to the b-axis. (C) 2002 Elsevier Science Ltd. All rights reserved.