29 species of limnetic rotifers were prominent in the open waters off Milwaukee, Wisconsin, July 1972 to June, 1973. Nine additional species were of rare or solitary occurrence. Eleven limnetic species were recorded for the first time in Lake Michigan. The predominant genera were Polyarthra, Keratella, Synchaeta and Notholca. Significant inshore-offshore differences in abundance and species composition were observed. The rotifer community in Milwaukee Harbor was composed mainly of benthic and littoral species as well as limnetic species characteristic of eutrophic waters. Pronounced vertical differences in distribution of species existed during thermal stratification. Rotifers were distributed during the late morning hours predominantly in the lower depths of the epilimnion and in the vicinity of the metalimnion. Currents and turbulence generated during high winds can distribute rotifers more uniformly throughout the epilimnion. The seasonal abundance of rotifers appears trimodal with two major peaks in July and September and a minor one in October.