Abstract The mean annual population densities of free-ranging sheep in upland North Wales were significantly correlated with near-infrared (0.76–0.90 μm) radiance recorded by the Landsat Thematic Mapper. The relationship between sheep density and the normalized difference vegetation index (NDVI) indicated that high sheep densities always occurred on vegetation with high NDVI values, but low sheep densities may occur on vegetation with either low or high NDVI values.