Members of the Performance Testing Division of the Merino Stud Breeders Association of South Africa select their young sheep according to a selection index, calculated from objective measurements of the following four characteristics body mass, clean fleece mass, fibre diameter and fold score. The average relative weighting factors for these four characteristics over the period of this study were 1: 10: -3: -3 respectively. Breeders can also measure the breeding value of their breeding rams against a genetic control flock. This test can be repeated in a particular flock at a later date to estimate breeding value changes over time. Tests were repeated after 7 years of index selection in three commercial flocks. The average percentage changes in breeding values for the different characteristics were as follows: body mass 12,1; greasy fleece mass 9,8; clean yield 3,9; clean fleece mass 14,4; staple length 10,6; crimps per 25 mm -5,3; fibre diameter -1,7; fold score -29,4. The responses obtained in the selected characteristics were in agreement with theoretical predictions.