Physiological resting cells (as opposed to resting spores or cysts) have been identified for the following freshwater diatom species: Actinocyclus normanii f. subsalsa, Asterionella formosa, Diatoma tenue var. elongatum, Fragilaria capucina, F. construens, F. construens var. venter, F. crotonensis, F. intermedia var. fallax, F. pinnata, Melosira granulata, M. islandica, M. italica subsp. subarctica, Stephanodiscus alpinus, S. binderanus, S. medius, S. niagarae, Tabellaria fenestrata and T. flocculosa. Resting cell populations were obtained from surficial sediments of three bays of the Laurentian Great Lakes. Surficial sediment resuspensions (down to 3 cm) were carried out in filtered bay water under lighted conditions which resulted in rejuvenation of these speces to growing vegetative populations. All resting cells were characterized by a dense cytoplasmic mass positioned in the center of the cell. Density of this mass varied between species. The ability of individual species to rejuvenate is affected by temperature, and probably nutrients and/or other environmental factors. Resting cell formation was studied in a unialgal culture of M. granulata isolated from a Douglas Lake resuspension. Resting cells appear as a function of culture age; however, their formation can be greatly accelerated by reduced temperatures and darkness. Our observations suggest that the ability to form resting cells and entrainment of such cells through turbulent mixing is an important factor in determining phytoplankton community structure and succession in the Great Lakes.