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The Mormon Genealogy Data Base has been used to compare longevity of twins with that of pairs of sibs in the same sibships. The data consisted of twins and sibs born between 1810 and 1899 whose Family Record Sheets were on file. We excluded all pairs in which at least one of the sibs or twins died in infancy.The findings show that twins are disadvantaged in comparison with their other sibs. Sib-sib life span associations were significant but lower than the corresponding correlations in all twin pairs, including the opposite-sex pairs. No significant associations were found between a twin and the next older sib. The special environmental constitutes of twins compared with sibs born to the same parents are corroborated by these findings. It is suggested that maternal mortality accounted for shorter life span in female compared with male twins. This relation was not evident when comparing the survivorship curves of female and male sibs.