The primitive sheep of North Ronaldsay Island (Orkney), which feed almost exclusively on seaweed, have developed physiological features linked to that diet. The use of seaweed to feed domestic animals, attested for centuries in north-western Europe, may have appeared soon after the arrival of the first domestic herds during the 5th and 4th millennia bc. The use of isotope analysis of sheep tooth enamel as a means to investigate seaweed grazing in prehistoric times was tested. The teeth of five modern North Ronaldsay sheep were analysed for carbon (δ13C) and oxygen (δ18O) isotope ratios. Owing to differences in the δ13C of marine vs terrestrial plants, the seaweed-eating sheep have δ13C values clearly outside the range expected for terrestrial plant eaters. Results show that one group of individuals relied exclusively on seaweed throughout the year. In a second group, terrestrial plants provided about half the dietary carbon pool during the summer. In individuals relying exclusively on seaweed, the amplitude of seasonal change in the δ18O values of tooth enamel was low, possibly because of ingestion of marine water through fresh seaweed consumption. Analysis of a sheep tooth from the Neolithic site of Holm of Papa Westray (Orkney, Scotland), suggests that as early as at the beginning of the 4th millennium bc in the Orkney, seaweed contributed to the winter diet of domestic sheep.