Flock genetics has a very major effect on the level and quality of output from the hill sheep sector. For example, ram breed substitution can increase output per ewe by up to 24% with concomitant improvements in carcass quality (Carson et al., 2001). Major opportunities now lie in improving the output potential of hill ewes through breeding. Currently, hill sheep flocks in the UK and Ireland consist mainly of purebred Scottish Blackface, Welsh Mountain, Swaledale and, to a lesser extent, Cheviot ewes. Using purebreds, rather than crossbred ewes, means that the potential benefits of heterosis are not exploited in harsh hill conditions where they are likely to be of greatest benefit. Crossbreeding can also be used to take advantage of breed differences in genetic merit for different traits e.g. hardiness, ease of lambing, prolificacy and carcass traits. In view of this background, a research programme was initiated to investigate the relative merits of a range of crossbred ewe types within hill sheep systems in Northern Ireland. This paper presents the results from the second phase of the study in which the performance of mature ewes (1-3 lambings) was investigated.