It has long been known that as a result of frequent deposition the mineralogy of Mississippi River sands remains constant in time and space along the river. There has never been a comparable, detailed mineralogical study of the vastly more abundant muddy sediment of the river. To evaluate the magnitude of variation in Mississippi River fine-fraction mineralogy, suspended sediment was collected at 3 stations across the river during July and November 1980. Samples were disaggregated, and without pretreatment, separated into 4 sizes: 62.5-7.8 mu m; 7.8-2.0 mu m; 2.0-0.49 mu m; and < 0.49 mu m. Using calibration curves prepared from pure minerals with an internal standard, XRD determinations of the mineralogy were obtained with a high precision. Minerals quantified were, in order of decreasing abundance: smectite, illite, kaolinite, quartz, plagioclase, alkali feldspar, chlorite, and amphibole. The largest source of variation in suspensate mineral abundance was sediment size. Smectite, for example, comprised 50% of the fine clay, but was not present as silt. ANOVA revealed significant temporal variation in the abundance of smectite and illite within the finest sizes and lateral variation in quartz and feldspar abundance in coarser sizes. Discriminant function analysis determined that the total mineralogy of river-suspended sediment varied significantly with time. This temporal variation in mineralogy apparently results from differences in soil mineralogy in drainage basins of the major tributaries of the river, coupled with the time-varying contributions of the tributaries.