This study originated from a graduate course in Great Lakes Limnology taught during the summer of 1970 at UMBS. It was the outgrowth of a seminar supported by the NSF and presented by the staff of the GLRD during the summer of 1968. ... Our study is unique in that we gathered extensive physical, chemical and biological information with uniform methods from four of the lakes within a 4-week period. Most reports treating the subject of Great Lakes eutrophication have been based on data collected by numerous investigators over several years and lacking uniform methodology. Our data include temperature, transparency, dissolved gasses, pH, alkalinity, major ions, nutrients, chlorophyll, carbon fixation by phytoplankton, phytoplankton, zooplankton and benthos. Our investigations were conducted in the summer of 1970 at 87 stations in Lakes Michigan, Superior, Huron and Erie.