The paper describes the thornbush invasion (bush encroachment) found in the savanna at Olifants Drift in eastern Botswana. Overgrazing by cattle is responsible for the opening up of the grass sward and has enabled woody species to establish. A quantification of this process between 1950 and 1975 in an area of ca. 100 km2 is given in terms of plant density as well as aboveground biomass. The loss of herbaceous biomass which generally accompanies bush encroachment and the possibilities for pasture restoration are mentioned.