Five British hill sheep breeds and their crosses were compared on a hill farm in a common environmental and husbandry system in 1974 in Peeblesshire, Scotland. A home-bred Scottish Blackface stock was maintained, and ewes were crossed with rams of each of the five hill breeds, North Country Cheviot, Derbyshire Gritstone, Exmoor Horn, Swaledale and Scottish Blackface. The crossbred female progeny were then mated half to homebred Blackface rams and half to rams of the paternal breed. This was reversed in later generations to form a continuous criss-crossing system. The performances of the crossbred ewes and of the crossbred lambs were compared, and the effects of the breeds and the average heterosis were estimated. A total of 5579 ewe and 6516 lamb records were available for comparison over an 8-year period from 1977 to 1984. Some 20 rams per breed were used over this period. The Swaledale crosses had substantially higher output and efficiency than any of the other crosses, while the Derbyshire Gritstone and Exmoor Horn crosses were generally inferior. There was appreciable heterosis for most traits, that for fleece weight and for litter weight at weaning being the highest. The heterosis for output per ewe exposed and efficiency of lamb production are positive and significant. The results support greater use of the Swaledale and of a crossbreeding system with Blackface to exploit heterosis in harsh hill farm conditions.