Interrill runoff and sediment yield from a topsoil having essentially textural pores and from another with structural pores were measured in the laboratory under simulated rain to investigate the effects of cover and positions of rock fragments. The results reveal that the relation between the cover of rock fragments and interrill runoff or sediment yield can be negative as well as positive. Porosity in the soil near rock fragments determines both the type of relation and the degree to which the positions of rock fragments affect this relation. Rock fragments increase runoff and sediment yield when well embedded in a surface seal (i.e. a top layer with essentially textural pore spaces). A negative relation occurs either where rock fragments are partly embedded in a top layer with structural porosity or where the rock fragments rest on the surface of a soil having either textural or structural pore spaces. The time at which macropores at the soil surface close is crucial, for then the effect of the cover of rock fragments on the generation of runoff switches from reduction to an increase. Since cover can have an ambivalent effect on the hydrological and erosion response, descriptions of topsoils containing rock fragments should include the type of soil porosity as well as the positions of rock fragments.