The increase in ovulation rate caused by the Booroola gene was first observed in the Booroola Merino strain of sheep (Davis et al., 1982) and the gene was subsequently mapped to sheep chromosome 6 (OOV6) (Montgomery et al. 1994). The low heritability of fertility traits and the desire to produce more lambs per ewe from meat breeds has lead to many crossbreeding programs seeking to obtain the benefits of the Booroola gene. However, many producers report animals carrying a Booroola allele to be lighter than non-carriers (G. Davis, personal communication). The Booroola Merino strain of sheep is typically lighter than recipient breeds used in the introgression programmes e.g. Romney. This study aims to determine whether the Booroola gene has a pleiotropic effect on liveweight or whether there is any evidence to suggest a closely linked quantitative trait locus (QTL) affecting liveweight that may ‘hitchhike’ with the Booroola gene.