Many soils have clay-enriched horizons that are thought to result from pedogenic processes. Various hypotheses are proposed for expected clay size-distributions in soil profiles. The average size of clay crystallites determined by the amount of broadening of x-ray diffraction peaks ranged from 160 to \u3e2000Aº , but the lack of trends in four loess-derived soils did not permit the hypotheses to be evaluated. An unexpected relationship in a well expressed illuvial horizon was noted: as the percent of accumulated clay increased to the observed maximum the average crystallite size also increased rather than decreased as proposed. This suggests that small-sized illuvial clay particles undergo reorganization to form larger stable crystallites. Knowledge of clay crystallite size is expected to supplement other data in interpreting soil genesis