Based on 0kl electron diffraction patterns of two terrestrial orthopyroxene samples, all k-odd reflections violating a b glide are present. Such reflections aro not caused by the overlap of diffractions from higher levels of orthopyroxene itsell or from the presence of C2/c clinopyroxene exsolution lamellae or G.P. zones, by difluse streaks along a*, or by the multiple diffraction effects. Observations of a hypersthene sample with a TEM equipped with a heating stage indicate that a phase transformation is produced in which all 0kl diffraction spots with k-odd systematically became unobservable when the sample was heated to approximately 900°C. The existence of a terrestrial orthopyroxene with space group P21ca is therefore confirmed.