We describe a shift in our understanding of cell patch dynamics in the surf diatom Anaulus birostratus (Grunow) Grunow as a result of recent findings concerning their vertical distribution. An extension of previous investigations to include the psammic or sediment habitat of the surfzone has shown this surf diatom to alternate from a planktonic and neustonic life mode during the day to epipsammic at night. The pattern and magnitude of this shift are discussed in the light of existing hypotheses, and form the basis of a new approach to the question of patch formation and decay by surf diatoms. Briefly, cells acquire a positive buoyancy during the day, enabling them to occupy the air/water interface despite their high specific gravity. This positioning allows for the build-up of a concentration gradient in the inner surfzone in spite of the excessive offshore drainage brought about by rip activity. The resultant concentration of cells at the surface is a major causal factor involved in patch formation. Evidence is provided to show that during the late afternoon cells switch adherence from air bubbles to sediment particles in order to enter the sediment. In this way the patch disappears but the cells are retained within the inner surfzone. The cycle is repeated when the epipsammic cells elute out of the sand in the early morning. In the absence of cell patches there is still a resident sediment population, although cell numbers are two orders of magnitude lower. We also observed that under these circumstances the elution process in the early morning was suppressed. Preliminary data obtained for other surf diatoms at the Sundays River Beach, South Africa, suggest the dynamics described above for A. birostratus to be common to this group of marine phytoplankton.