The clay fractions of 31 samples of Neuse River sediments were analyzed by x-ray diffraction techniques in order to determine their clay mineral composition and to determine the diagenetic changes in a stream environment. Kaolinite, illite, montmorillonoids, "chlorite," and mixed-layer aggregates were identified. Kaolinite is the dominant mineral but decreases downstream. Illite occurs sporadically throughout the length of the stream. Montmorillonoids occur in small amounts in the upper part of the stream. A chloritic mineral is found in the lower part of the stream and in the upper part of the estuary. This mineral is similar to chlorite in that it gives a 14 A basal spacing that does not shift with glycerol or ammonium chloride saturation but dissimilar in that the 14 A spacing is altered by heating, at 550 degrees C. Mixed-layer aggregates, possibly of illite and "chlorite," occur throughout the length of the stream but increase downstream. Material, amorphous to x-rays, is common or abundant in nearly all of the samples. It is probable that kaolinite and montmorillonoids are partly altered to amorphous material, "chlorite," and mixed-layer aggregates.