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An enteric disease characterised by diarrhoea and ill thrift affected 12 of a flock of 700 six- to 12-month-old ewe lambs in Cornwall between December 1996 and September 1997. The affected lambs were undersized, became thin and suffered an unremitting diarrhoea until they died. The illness lasted for three to 14 days, although, with hindsight, the owner considered that the lambs had been below average size before the enteric signs developed. The outbreak ceased only as a result of the dispersal sale of the flock as breeding ewes. The flock had been purchased from different sources, but 11 of the cases occurred in a group of 40 purchased from one source. Postmortem, the alimentary changes resembled mucosal disease in cattle, and immunostaining of histological sections of the affected tissues revealed pestiviral antigen. Non-cytopathic pestiviruses were isolated from the lesions of two of the affected lambs and from the blood of several clinically normal ewe lambs from the same group. All the pestivirus isolates were typed as Border disease virus.