Selected area and convergent-beam electron diffraction have been performed on LaGaO3 at room temperature and 220 degrees C and LaGa0.9Mg0.1. O-2.95 at room temperature. It has been experimentally established for the first time that the space groups of the orthorhombic LaGaO3 at room temperature and rhombohedral one at 220 degrees C are Pbnm (No. 62) and R(3) over bar c (No. 167), respectively. Crystal system of LaGa0.9Mg0.1O2.95 at room temperature has been settled to be orthorhombic with space group of Pbnm (No. 62). Existence of a long-period anti-phase domain structure in LaGa0.9Mg0.1O2.95 has been indicated from splitting and broadening of selected area electron diffraction spots. (c) 2007 Elsevier B.V. All rights reserved.