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The roles of edaphic factors and competition in explaining species distribution of Pteronia (family Asteraceae) dominated plant communities, were investigated. Soil characteristics, species plant covers and competitive interactions were determined at three sites (one on a colluvial plain and two on tillite slopes) on the farm Tierberg in the semi-arid Karoo, South Africa. These sites represented areas where Pteronia species replace each other in apparently distinct habitats. Direct (Canonical Correspondence Analysis, CCA) and indirect (Detrended Correspondence Analysis, DCA) gradient analyses gave similar eigen values, indicating that edaphic variables accounted for much of the variation in species data. The first axis of the CCA-biplot separated species on the colluvial plain from those on tillite slopes along a soil texture gradient. The separation of species in the second CCA axis indicated that soil phosphorous, pH, magnesium and calcium levels were also important in determining species distribution patterns. Together, these axes accounted for 50.1% of the variation in the species data. Nearest-neighbour analyses indicated that there were strong inter- and intraspecific competitive interactions at Pteronia community boundaries. If edaphic specialization alone was responsible for Pteronia distribution patterns, one would expect there to be reduced competition at the boundaries of species overlap. It is concluded that Pteronia species distribution patterns in the Karoo result from the combined effects of soil physico-chemicalfactors and competition.