X‐ray diffraction topography can be used to describe ultrasonic vibrational modes in resonating crystals. Dependence of intensity enhancement on Bragg reflection and on the vibrational displacement parallel to the diffraction vector allows the determination of the polarization direction and positional details of the standing waves to be made with four or five topographs in simple cases. In an SL‐cut quartz resonator designed to operate at 455 kHz the principal mode is thereby shown to be third‐order fac