A method of estimating the quantity of biomass in the ground layer of semi-arid rangeland is described and applied to a region of 700 km2in Botswana. Vertical true colour diapositives, each covering approximately 60 × 90 m, were taken at 1 km intervals from a light aircraft flying at about 120 m above the ground along transects 1·6 km apart. The results of four seasonal surveys are presented. The ground layer cover, as a percentage, was estimated on each diapositive. It ranged mainly from 10 to 60 percent. A set of sampling plots on which the cover and biomass were measured on the ground provided standards for the air photograph estimates and a calibration equation of exponential form from which to convert estimates of cover to those of biomass. The values obtained were analysed spatially. Sample semi-variograms were computed and modelled as anisotropic power functions to lags of 10 km by least squares approximation, and validated by kriging. The grass cover and biomass were mapped from the sample data by first interpolating a figure field of cover on a fine mesh grid by kriging, and then contouring the figure field. The maps were converted to ones of biomass using the exponential calibration equation. A strong regional pattern emerged that remained fairly constant at different seasons and that matched the pattern in the underlying geology. Four fairly distinct sub-regions were delineated, and the average cover and total biomass were estimated, again by kriging, for each seasonal survey. Biomass in the sub-regions ranged from 185 to 1700 kg/ha.