piezoelectric resonator comprising a sealed box in which a vacuum has been formed or into which has been introduced a residual gas , within said box a piezoelectric crystal having first and second main opposite faces , a first metal electrode deposited on the first main face of the crystal and a second metal electrode deposited on the second main and opposite faces of the crystal , a first conductor connected to the first electrode and extending outside the box , a second conductor connected to the second electrode and extending outside the box and means for maintaining the crystal within the box , wherein the piezoelectric crystal comprises a central portion and a peripheral portion forming a ring which spacedly surrounds the central portion and connected to said central portion by an intermediate portion which over at least part of the distance between the central portion and the peripheral portion has a reduced thickness compared with the thickness of said central and peripheral portions wherein the first and second electrodes are deposited on one of said central and peripheral portions which then constitutes the active part of the crystal , while the other portion which does not carry electrodes is immobilized in the box by retaining means which act in localized zones , the intermediate portion of the crystal and said immobilized portion constituting the suspension means for the active part of the crystal .