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The effects of the activity of seed harvesting ants (Messor bouvieri) on the fertility, rainfall infiltration, structural properties and water repellency of top soils were investigated in a semi-arid rangeland in SE Spain. The soil surfaces had a low vegetative cover and bare areas had a sieving crust. Rainfall simulation experiments were carried out over ants’ nest mounds and on control areas without surface ant activity in September 1997 and October 1998. The soils of the ants’ nests had a lower pH, higher concentrations of organic carbon and inorganic nutrients, higher structural stability and were more water repellent than the control areas. Infiltration rates on the control areas were comparable in both years. However, in 1997, infiltration was significantly higher on the nests than in control areas, whereas in 1998, infiltration was lower and wetting depth was reduced on the nests. These contrasting results are explained by a difference between the 2 years in the initial soil moisture content and the water repellency of soils and organic debris on the ant-affected areas. It is concluded that ants’ nests can act as sinks for water under slightly humid to wet conditions, whereas under extremely dry conditions, which prevail in summer and the beginning of autumn, infiltration is strongly reduced. Temporal variability in the initial conditions of the soil and spatial variability in ant activity interact to influence runoff generation at a fine scale and should be taken into account to fully understand runoff generation at the hillslope scale. The redistribution of materials by ants and their effects on soil properties may have consequences for the development of fertile islands in semi-arid ecosystems. (C) 2002 Elsevier Science B.V. All rights reserved.