X-ray diffraction determined clay mineral crystallinity data, standardized to the Crystallinity Index Standard (CIS) scale, is presented for illite (white mica)and chlorite-bearing pelitic rocks of the zeolite, prehnite-pumpellyite, pumpellyite-actinolite and the greenschist facies. Good correlations exist between both illite and chlorite crystallinity and crystallite size data, with respect to the mineral (metabasite) facies conditions, using both the Full-Width-at Half-Maximum and a Siemens Single-Line Fourier method, while calculations employing the Warren-Averbach method yielded unacceptably high degrees of error, and hence poor correlations. these preliminary data conform with the commonly held view that the diagenetic-, anchi-, and epi-zones of Kublers illite crystallinity boundary limits, adopted by the CIS, correspond with the zeolite, prehnite-pumpellyite-actinolite, and greenschist facies, respectively. The CIS scale is therefore considered a reliable indicator of metamorphic grade, when adequate attention is given to the calibration of the experimental data and the selection of suitable peak reflections. The air-dried Sr-or Ca-saturated illite 002 and chlorite 003 peak combination are recommended as two useful reflections for accurate and rapid XRD grade determinations in very low-grade metamorphic rocks.