Approximately 6O bottom sediment 'grab' samples were obtained in the south basin of Lake Winnipeg in the autumn of 1969 and spring of 1970. Textural analyses indicate that most offshore sediments contain greater than 70 percent clay-sized minerals (finer than four microns). The sedimentary units within the basin occur in three concentric units: a large clay unit in the center of the basin, a smaller silty clay unit 'shoreward' of the clay unit and a nearshore undivided unit consisting of sand, gravel, underwater 'outcrops' of glacial till and Glacial Lake Agassiz sediments and a thin discontinuous layer of silt and clay. X-ray analysis of magnesium saturated, glycerol-treated unoriented slides of the fine clay fraction shows the mineralogy of the recent bottom sediments to be essentially homogeneous throughout the lake. The clays consist of approximately 80 to 85 percent randomly interstratified montmorillonite-illite and 10 to 20 percent kaolinite plus chlorite. The proportion of illite in the interstratified complex is 45 to 50 percent. The montmorillonite and illite are both dioctahedral. Differential thermal analysis indicates that the montmorillonite-illite interstratification consists of 'normal illite' - 'abnormal' montmorillonite with the direction of transformation being illite converting to montmorillonite. The dominant exchange ion on the montmorillonite is calcium. The mineralogy of the Lake Winnipeg sediments agrees, in general, with that found by other researchers for the Cretaceous Shales, surface deposits and soils of Western Canada; and from this it would appear that little or no diagenetic changes have occurred in the clay minerals from the Cretaceous Shale source through the sequence of glacial erosion and redeposition as tills and glacio-lacustrine clays, pedogenic processes of soil formation, erosion of the soil and finally to deposition in Lake Winnipeg...