A technique is described for determining the equivalent circuit ot a crystal resonator by measuring its frequencies of series and parallel resonance, its insertion loss as a series element in a suitable low-impedance transmission test-set and its capacitance at 1 kc/s. The effect of secondary responses on the equivalent circuit is discussed. A new test holder is used in which, with the aid of a microscope, specimens can be mounted in clamps at any desired point on their major surfaces. The results of measurements obtained on a number of types of quartz, ammonium-dihydrogen-phosphate and ethylene-diamine-tartrate resonators are given in tabular form. Illustrations of the effect of off-node mounting are included.