(1) A number of plots were set up in two natural savanna communities, with identical climates but different soils, to examine possible competition between the woody and herbaceous components of the vegetation. The community on the more sandy soil is a broad-leaf woody savanna and the other a more open microphyllous Acacia community. (2) Vegetative growth and soil water were monitored over a 2-year period in control plots and in plots cleared of one of the vegetation components. (3) In the broad-leaf community the effect of the herbaceous vegetation on the woody plants is negligible. (4) In the Acacia community with seven times more herbaceous biomass, mature woody-plant growth was reduced by competition from the grass-dominated herb layer particularly in the first (wetter) year. The vertical root distributions and soil-water data indicate that the grasses take up topsoil (0-30 cm) water sufficiently rapidly to reduce drainage into the subsoil (30-130 cm). and that they also take up subsoil water directly, thus lowering the amount of subsoil water available to woody plants. (5) The different herbaceous to woody-plant biomass ratios in the two sites and the different intensity of competition during the 2 years can be explained in terms of the effects of the soil properties and of the rainfall intensity on the ratio of water in the topsoil to that in the subsoil. © Blackwell Science Ltd. Abstract reproduced by permission.