An experimental programme was initiated in 1990 to investigate the consequences of selection for reduced fatness in hill sheep. Lambs from some hill breeds tend to fall at the poorer end of the carcass quality scale, and achieve low carcass weights due to their small mature size. The aim of this paper is to compare carcass characteristics of lambs sired by two lines of Scottish Blackface rams selected for divergent subcutaneous backfat, and to estimate heritabilities at three ‘end ’ points: 1) at a constant subcutaneous backfat level, 2) at a constant age and 3) at a constant cold carcass weight.