The preweaning growth and carcass characteristics offour lamb genotypes were analysed for variation attributable to the intrauterine cohabitant (IUC) phenomenon, where within-sex variation is attributed to the sex of a twin's womb-mate. The four genotypes resulted from the following matings: Poll Dorset rams × Merino ewes, Poll Dorset rams × (Border Leicester × Merino) ewes, Texel rams × Merino ewes and Texel rams × (Border Leicester × Merino) ewes. Four hundred and twenty twin lambs participated in the preweaning study and of these, 209 were slaughtered to generate carcass data. Some of these genotypes displayed within-sex variation in weaning weight and fat score, preweaning average daily gain, skin-fold thickness, carcass fatness and eye muscle dimensions attributable to the IUC phenomenon. Genotypic differences in the responses of lambs to the IUC phenomenon were also observed. The IUC phenomenon appears to involve prenatal programming, an hypothesis that attributes postnatal characteristics to events during differentiation. The IUC phenomenon is analogous to the intrauterine position phenomenon in fecund mammals, where variations in prenatal steroid concentrations programme for permanent alterations in postnatal reproductive characteristics. The growth responses reported in this paper provide evidence of variation due to prenatal programming. While the magnitude of the responses to the IUC phenomenon were not large, the data presented indicate that under field conditions, the magnitude of growth responses to the IUC phenomenon may be as great as those observed between breeds. If so, further examination of the role that an animal's IUC may have on its subsequent performance could aid the development of more sensitive indices for breed evaluation and progeny selection.