Transition from a flat, two-dimensional community to an erect, three-dimensional community has qualitatively been observed during diatom colonization on artificial substrates. To determine if the same progression occurs on natural substrates, epipsammic diatom communities were examined. Acid-cleaned sand grains were placed in a plexiglass collecting chamber and exposed to colonists in the west branch of the Maple River, Emmet County, Michigan between 29 June 82 and 1 August 82. Sand grains were collected with a coring device every 4 days beginning 4 July 82 and processed for light and scanning electron microscopy. Three-way analysis of independence revealed that epipsammic diatoms were significantly associated with cracks and valleys of sand grains. Chi-square analysis indicated that stalked diatoms, a minor component of the community, were randomly distributed on sand grains. Cluster analysis indicated that communities collected over the 33 day period were structurally very similar to each other (SIMI = 0.80). Two-way analysis of variance showed that the most common diatom habit was a flat, prostrate form while absolute abundances of diatoms on sand grains were significantly different on day 5 only. A non-significant interaction term indicated that diatom communities did not change from a flat, two-dimensional community to an erect three-dimensional community with time. Relative abundances were exceptionally low with the dominant species, Cocconeis diminuta, representing only 14% of the community. The epipsammon represent a specialized diatom community that seems well adapted for existence in a variable environment. Disturbance probably plays an important role in structuring the community and as a result community changes are only those of self-replacement or autosuccession.