An exceptionally high incidence of difficult birth (29% of ewes giving birth to single lambs) in a Cheviot sub-flock in 1957 was reported by Gunn and Robinson (1963). To analyse some of the factors contributing to variation in the incidence of difficult births, observations were made on three farms over a seven-year period from 1958 to 1964. The farms are Sourhope, Roxburghshire, in the Cheviot Hills, with 1000 Cheviot and 350 Blackface ewes on predominantly Festuca-Agrostis, Nardus and Molinia pastures, with no regular provision of supplementary feed; Glensaugh in the East Grampians in Kincardineshire, with 550 Blackface ewes on predominantly heather (Calluna) pasture on free draining ground, with regular provision of supplementary feed during winter and spring; Lephinmore on Loch Fyne in Argyll, with over 600 Blackface ewes on mixed pasture mostly overlying peat, with limited provision of supplementary feed during winter and spring. Further details are given in the H.F.R.O. report (1958).