The Straits of Mackinac, from the standpoint of physical dynamics, is a unique area in the Laurentian Great Lakes. It is unique in that the Stratis connect two lakes with the same water level, and although there is a net water transport from Lake Michigan to Lake Huron the flow oscillates between the two lakes. Outflows at other points in the Great Lakes system are due to differences in water levels. The oscillatory flow resulting from the connection of two lakes with the same water level would be expected to produce complicated physical dynamics and possibly a unique biological environment. The physical processes have been studied infrequently. Saylor and Sloss (in press) measured currents and water movements in the Straits during the time our study was conducted. The biology and ecology of the Straits of Mackinac, northern Lake Michigan and northern Lake Huron are poorly known. The benthos has been studied by Henson (1962, 1970). To our knowledge there have been no investigations on the plankton–the limited data available are reviewed in Sections VI and VII, which present our results on phytoplankton and zooplankton. Likewise, little is known about the phytoplankton productivity and major nutrients in the Straits of Mackinac. Indications of accelerated eutrophication have been reported for Lake Michigan in recent years (Beeton 1969; Schelske and Stoermer 1971), but the impact of inputs of Lake Michigan water on eutrophication and primary productivity in the receiving waters of the Straits of Mackinac and Lake Huron has not been assessed. A review of the biological and chemical conditions in relation to the eutrophication and trophic status of Lake Michigan has been completed recently (Schelske, in press). Our study was initiated in late August 1973 with data being collected on three cruises: 30 August-1 September, 16-18 September and 6-8 October. The purpose of this investigation was to gather baseline data on environmental quality in the Straits of Mackinac and to use these data as outlined in the objectives.