The spatial variability of the factors of the universal soil loss equation is examined on the mediterranean basin of Conca de Tremp covering 43.1 km2in Spain. The evaluation of the rainfall erosivityRand the soil erodibilityKis relatively straightforward and spatially-averaged values of these parameters can be applied to the entire basin. Conversely, the spatial variability of annual soil erosion losses on large basins depends primarily on the factorsL, SandCdescribing topographic, vegetation and land use parameters. A grid size analysis of soil erosion losses from the Conca de Tremp basin under mediterranean climatic conditions in Spain shows excellent agreement with the earlier results on the Chaudière basin in Canada. It is concluded for both basins that unbiassed estimates of soil erosion losses are obtained for grid sizes less than about 0.125 km2. The analysis of the Conca de Tremp basin validates the use of the grid size factor proposed by Julien & Frenette (1987). It is also found that the grid size factor primarily depends on the average slope gradient which decreases with increasing grid size or drainage area. On the other hand, the grid size factor does not depend on the spatial variability of the factorsR, K, LandC.