The vegetation of flat-top mountains (mesas) was investigated in northwest Namibia. The main questions were: (1) Are there differences between vegetation types on mountain tops and what is the composition of mountain top vegetation? (2) Do rare species affect the floristic relationship between mountain tops? and (3) What are the likely factors determining the vegetation on mountain tops? Seven vegetation types were described, comprising a sparse to medium cover mix of mainly shrubs and grasses. Vegetation types differed remarkably between mountain tops, even within the proximity of a few kilometres. No single environmental driver can explain the vegetation on the investigated mountain tops. Biogeographic influences (distance to other mountain habitat, size of habitat per se, position within phytogeographic zones), as well as physical variables such as soil properties, elevation (height above surroundings), a westeast gradient of increasing rainfall, and possible rain shadows were considered of importance in determining the documented vegetation types. Keywords: Arid lands, Biodiversity, Biogeography, Endemism, Flora, Herbivores, Inselbergs, Mesa, Namib desert, Southern Africa.