The most abundant plankton diatoms in Lake Michigan have characteristic distribution patterns which appear to be largely controlled by their response to trophic factors. Consideration of the abundant occurrences of 44 taxa in 320 samples suggests several general patterns. Some taxa appear to be abundant only in highly oligotrophic habitats. Several taxa are perennial dominants only at offshore stations, and most of these show extreme morphological adaptation to planktonic existence. Many of the dominant taxa in Lake Michigan are eurytopic and occur in abundance at all except grossly disturbed stations. Taxa usually reported from smaller eutrophic lakes are common at some nearshore stations and in Green Bay. A number of taxa which appear to be especially characteristic of disturbed situations in large lakes are abundant in our most recent samples. Comparison of modern and historic records indicates that entities in the latter three categories, especially the last one, are rapidly increasing in relative abundance in Lake Michigan.