Diatom assemblages on limestone cobbles collected from 42.7 m depth in the mid-lake high region of Lake Michigan are characterized by low total abundance and low diversity. The flora present contains three associations. A large aloochthonous component, comprised of planktonic species and very low numbers of benthic species usually found in shallow water is present. A limited number of dominant taxa appear to be particularly associated with deep habitats in oligotrophic lakes. Other species, usually reported from terrestrial habitats, appear to be associated with the bryophyte Fissidens fontanis which is present in all collections.