X-ray diffraction studies of cultured siliceous diatoms indicate that poorly ordered strained quartz is the major identifiable mineral phase at the time of death, rather than incipient cristobalite as assumed by previous studies. Cultured speicmens of the freshwater diatoms Stauroneis anceps , Cyclotella meneghiniana , and Navicula sp. all gave X-ray powder patterns of alpha -quartz, although the lines were broadened and very weak. These diatoms contain about 99% amorphous material and about 0.l to 1% strained alpha -quartz, in comparison to the peak intensity of normal quartz. An index of quartz crystallinity based on the peak area/gram for the (1011) reflection of alpha -quartz (26.6 degrees 2theta , Cu Kalpha ) was used to relate quartz in cultured diatom frustules to natural samples of opal, quartz, and chert. The index of quartz crystallinity varied over three orders of magnitude from opal (0.001) to quartz (1.1) with diatoms (0.012-0.11) being intermediate. Unit cell determinations of alpha -quartz in diatom frustules indicate that the quartz is strained. Aging or heating the diatoms seems to relieve this strain, at the same time increasing the crystallinity of the quartz making up the frustules. These changes are probably related to water loss.