Soil samples were collected from 24 sites along a precipitation transect in semi-arid and arid southern Africa (Botswana and Namibia), ranging from approximately 550 mm average annual rainfall to less than 50 mm and grouped into four precipitation classes. The objective was to determine whether soil microbial biomass and activity mainly in these classes reflects changes in water availability or to some extent changes in soil chemical properties and texture. The texture of the soils belonging to the four precipitation classes was generally dominated by sand. In the wettest precipitation class (>400 mm), mean contents of soil organic C (5.6 mg g⁻¹ soil), total N (0.51 mg g⁻¹ soil), and microbial biomass C (112 μg g⁻¹ soil), as well as the mean basal respiration (5.6 μg CO₂–C g⁻¹ soil d⁻¹) were always highest, exceeding the mean values of the driest precipitation class (<50 mm) 4- to 8-fold. However, the mean microbial biomass C/soil organic ratio reached the highest value (4.3%) in this latter class. Soil organic C, total N, and basal respiration all showed highly significant correlations with the mean annual precipitation. Soil organic C and total N, but not basal respiration, were also significantly correlated with clay content and cation exchange capacity. Microbial biomass C revealed a strong correlation with total N, but especially with soil organic C. Small changes in clay content are just as important as the mean annual precipitation in the present sandy soils. However, significant amounts of soil microorganisms exist even in the driest areas. Keywords: basal respiration, Kalahari,  microbial biomass C, Namib, semi-arid and arid environment,  soil organic C.