Western Lake Superior water, which is used for municipal water supplies, contains large concentrations of asbestiform amphibole fibers because of a taconite tailings discharge at Silver Bay, Minnesota. Large fluctuations in fiber concentrations are attributable to seasonal and meteorological changes which influence Western Lake Superior water circulation. Variations in electron microscope amphibole fiber counts generally correspond to variations in amphibole mass concentration determined by x-ray diffraction. Daily x-ray diffraction and occasional transmission electron microscope measurements of Duluth, Minnesota drinking water samples allows an estimate of the average (45-100 million) and the range (10-1000 million) of identifiable amphibole fibers per liter of water. Not all the amphibole fibers present can be identified due to their extremely small width. Copyright (c) 1976 by The Institute of Electrical and Electronics Engineers, Inc.