Toman's theory of the relation of birth-order compatibility and interpersonal relations was extended to dyads (pairs) with members of the same sex. It was predicted that interpersonal compatibility between members of same-sex dyads (50 female, 27 male pairs of Vassar College roommates) would be positively correlated with birth-order compatibility (no rank or sex conflict) of the dyad members. That is, it was expected that the oldest sister of sisters would get along better with the younger sister of sisters (no rank or sex conflict) than with the older sister of brothers (rank and sex conflict). All pairs indicated (a) their familial birth-order relations, and (b) the degree of conflict experienced with their roommates, rated on a 5-point scale. Results supported the prediction. A second study (31 female, 31 male pairs) replicated these results, and indicated that this extension holds for same-sex dyads across both sexes. A discussion of the place of Toman's theory in birth-order research is offered. (Author/PFS)