Diatoms were collected in four areas of Lake Michigan and in southern Green Bay from April to early November 1965. The flora of the lake was characterized by Fragilaria crotonensis, Tabellaria flocculosa, Melosira islandica, Cyclotella 'glomerata-stelligera,' Cyclotella michiganiana, Asterionella formosa, and Stephanodiscus tenuis. Stephanodiscus niagarae always dominated the diatom biomass in Green Bay; also characteristic of the bay were Melosira granulata, Melosira ambigua, Melosira binderana, Fragilaria capucina, and Fragilaria crotonensis. There was an average of 517 diatoms per ml in the lake and 944 in the bay. Diatom abundance appeared to have one peak in the lake and two in the bay. At times of maximum abundance, optimum depth was usually 5 m. Numbers were greater in Green Bay and the inshore areas than in the offshore areas and appreciably higher near the eastern shore than the western shore of the lake. The higher populations near the eastern shore and in Green Bay appeared to be correlated with higher concentrations of phosphorus and chlorophyll a and lower concentrations of nitrate-N.