The Romney sheep group breeding scheme was initiated in 1979. Twelve pedigree breeders formed a Nucleus flock (N), at Wye College, each contributing ten “elite” ewes identified as prolific. The minimum entry requirement was that each ewe should have given at least twins at the previous lambing. The elite ewes were then performance tested in a common environment and those failing to give a multiple litter were replaced by other candidates from the flock of origin. The performance test was discontinued in 1982 when N was closed. Male and female replacements were chosen from the N-bred progeny. Replacements were selected on the basis of dam and sire's dam prolificacy record (live lambs born/ewe joined LLB/EJ)); ten ram lambs were chosen each year and thirty to forty ewe lambs. Ram lambs were used once as sires. Selection was for prolificacy with independent culling levels for tendency to prolapse, udder shape and breed type. A control flock (C) was set-up in 1982 by sampling at random within flock and age the twelve scheme member's flocks. Fifty pregnant ewes were assembled prior to the 1982 lambing. N and C were managed together apart from mating. No selection was applied to C.