This article uses samples of siblicide from Canada, Great Britain, Japan, and Chicago to explore the possible relevance of seniority in siblicide. The tendency for the killer to be the younger party was especially true of cases in which victim and killer were same-sex adults and, especially, brothers close in age. The older party was much more likely to be the killer when one or both were children, but this tendency is adequately accounted for by the changing age-specific likelihood that one will commit a homicide at all. Only the Japanese data set contains information on actual birth orders, which were not demonstrably related to the likelihood of either killing or being killed by a sibling. An analysis of the Canadian data suggests that the rate of siblicide is unaffected by the age difference between siblings. The substance of lethal sibling conflicts is discussed in the light of these results, case descriptions, and literature on nonlethal sibling conflict.