The literature was examined to relate ambient silica concentrations to numerical dominance of specific freshwater planktonic diatoms. Mean silica concentrations (mg/liter) during dominance are: Stephanodiscus astraea, 0.6; Tabellaria flocculosa asterionelloides, 0.9; Asterionella formosa, 1.7; Melosira granulata, 13.4. The concentration at time of dominance and patterns of diatom periodicity observed in many eutrophic lakes led to the hypothesis that declining ambient silica concentrations may influence the sequence of seasonal succession. When silica becomes depleted diatoms become scarce. It is suggested that some measure of “silica demand” could be used as an index of increasing eutrophication.