Soils with relatively sandy topsoils over clayey subsoils are widespread, but their genesis is not always clear. We tested the hypothesis that earthworms stimulate the formation of the sandy surface soils by producing clay-rich worm casts that are susceptible to erosion, in an undisturbed forested watershed in southwestern Ivory Coast. Soils of the watershed were strongly weathered and acidic. Their contents of ironstone gravel and of clay, organic matter and plant nutrients decrease in a downslope direction. Production of worm casts increased in the same direction from 0.1 to 0.2 kg m−2 yr− near the top of the watershed to between 2.3 and 9.6 kg M−2 yr− near the valley bottom, with an areal average of 3.2 kg m−2 yr−. Casts were invariably richer in clay and silt, organic matter and N, P, Ca and Mg than nearby bulk surface soil. Casts were easily disintegrated by rain splash and overland flow and cast material contributed to the 0.12 kg m−2 yr− of highly organic suspended sediment removed by surface water drainage. Selective erosion of worm casts was probably more important than eluviation of clay in removing clay from the surface soils. Increasing production and disintegration of casts with decreasing elevation probably played a major role in lowering contents of clay and nutrients in the surface soils in the lower parts of the area.