Transition from a flat two-dimensional community to an erect three-dimensional community has been described primarily for diatom colonization on rigid, artificial substrata. To determine if the same community transition occurs on natural, unstable substrata we examined colonization on sand grains over 33 days using both light and scanning electron microscopy. Epipsammic diatoms were significantly associated with crevices on sand grains and stalked diatoms, a minor component of the community, were randomly distributed. Diatom communities collected at different times over the period were structurally very similar to each other (similarity index SIMI=0.80). Diatom colonization did not develop from a flat, two-dimensional community to an erect three-dimensional community. The epipsammon represents 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 keeps it in a ’pioneer’ state.