In cattle, nearly all heifers born co-twin to a male are XX/XY chimeras exhibiting a characteristic freemartin phenotype. However, in sheep where mixed-sex litters are common, freemartins are relatively rare. The purpose of this study was to determine if XX/XY chimerism existed undetected in female Rideau Arcott sheep. In addition, breeding records were used to investigate the effect of litter size, litter sex ratios, and heredity on the incidence of chimerism in these lambs. Animals were screened for male DNA via PCR analysis of DNA obtained from peripheral blood. Of 230 lambs screened, 10 were identified as chimeras. Litter size and sex ratios had no significant effect on the incidence of chimerism, lending support to a hereditary component. Furthermore, PCR and FISH analysis of 4 chimeras revealed distribution of XY cells to tissues outside the blood. SRY gene expression was also detected in the gonads of one animal.