TheQ(quality factor) of natural quartz was measured from 4.2°K to over 100°K. The quartz was in a high precisionATcrystal unit vibrating on overtones of the thickness shear at about 6.3 mc and 8.8 mc.At higher temperatures theQvaried rapidly with temperature, but did not depend on the particular mode of vibration.At low temperatures (say less than 10°K) theQalso depended on the mode of vibration. At these low temperatures theQbecame very large—a record of 55 million was observed on one mode at 4.2°K. Apparently another loss mechanism dominates at low temperatures, and very likely this mechanism may not be intrinsic to quartz. The mounting structure or gold electrodes may be the primary source of energy loss in the liquid helium temperature range.