Ant occupancy in thorns of Vachellia (Acacia) karroo (Hayne) Banfi and Galasso was examined along the banks of the Great Fish River near Grahamstown, South Africa. Half of all thorns with visible openings contained ant nests. Colonies of Tetraponera emeryi (Forel) and a single unidentified Tapinoma species were present in over 40% of thorns with openings. Cataulacus intrudens (Smith, F.) was regularly encountered but at a much lower frequency. A single nest of Tetraponera natalensis (Smith, F.) and two Nesomyrmex stramineus (Arnold) nests were also found. The size of contiguous patches of A. karroo was not related to the percentage of thorns occupied by ants. The A. karroo ant assemblage appears to be mediated by the availability of suitable nesting thorns rather than competitive interactions.