The clay minerals were studied from eleven soil profiles representing three Great Groups of eastern and central Oklahoma. Montmorillonite and illite type clay minerals were found to be the dominant minerals; montmorillonite dominates the fine clays; illite is the chief constituent of the coarse clays. The fine clays dominated the clay fraction of most horizons, including the parent material. The results of the investigation indicate that the present régime of soil formation has had little effect in altering the clay minerals of the soil from those present in the original sediments.