Interactions between perennial grasses occupying the sides of sand dunes on the E. edge of the S.-central Namib desert were studied using nearest-neighbour analysis. The species studied were Stipagrostis sabulicola, Cladoraphia spinosa, S. (cf.) namaquensis, S. ciliata, S. lutescens and Centropodium glaucum. There were strong intraspecific competitive interactions within a vegetation zone and interspecific interactions where one species replaced another over the dune slope. Interspecific nearest neighbours were closer to each other on the upper dune slope than intraspecific nearest neighbours. On the lower slopes, where the sands were more stabilized, tolerances of the various species were more complex. Rates of water uptake by the different species after watering with the equivalent of a 10 mm rainstorm were related to their rooting depth and position on the dunes. It is suggested that there is a cyclical succession on the upper dune slopes. The sand is stabilized by the mound-building S. sabulicola, which is then replaced by S. namaquensis; this does not survive burial and is in turn replaced by S. sabulicola. On the more stable lower slopes morphologically similar species coexist by habitat partitioning and morphologically dissimilar species coexist as a result of root zonation and different seasonal patterns of water utilization.