This report presents comprehensive data on species composition and distribution of planktonic rotifers in Saginaw Bay and southern Lake Huron from April to November, 1974. Rotifer species composition and abundance differed greatly between Saginaw Bay and open Lake Huron waters. Through cluster analyses, these differences were empirically related to the physicochemical environment. The results of these analyses suggest that rotifers are valuable organisms in water quality assessment studies. Several species which displayed distribution limited to eutrophic Saginaw Bay stations or to oligotrophic offshore Lake Huron stations were potentially useful as environmental indicators. Based on rotifer data, the greatest impact of Saginaw Bay waters on Lake Huron occurred along the western shore of southern Lake Huron below the mouth of the bay. In general, inshore stations of southern Lake Huron displayed greater rotifer abundance than mid-lake stations. Certain rotifers displayed distinct epilimnetic or hypolimnetic vertical distributions. However, maxima of total rotifer abundance usually occurred in the vicinity of the metalimnion. Wind-generated turbulence often distributed rotifers more evenly in the epilimnion.