Depth distributions of epipelic diatoms and dissolved oxygen in agricultural stream sediments were studied. The major fraction of the diatoms were found in the top 2 cm of sediment. The distribution of diatoms and dissolved oxygen are influenced by each other. Larger numbers of cells were found at depths where oxygen concentrations were high. However, dissolved oxygen profiles measured in situ indicated that a large part of the sediment where viable cells are found is anoxic. In most cases anaerobic conditions exist below 7 and 4 mm depth during day and night respectively. Laboratory experiments suggested that under anaerobic conditions not only was there no evidence of growth but also that mortality increased with increasing depth;In contrast to lake and marine sediments diurnal migration patterns were not observed in this study. Migration and distribution patterns may have been strongly influenced by the more turbulent nature of the stream environment. Great numbers of cells might be lost from their site of production and rates of production might be much higher than would be indicated by the low cell densities routinely observed in situ. Daytime population densities were much higher than nighttime. These differences in population densities were due to a balance of growth, mortality, and loss of cells from the site of production.