Intrasexual competition implies viewing the confrontation with same-sex individuals, especially in the context of contact with the opposite-sex, in competitive terms. After constructing the items for the preliminary scale and after conducting a pilot study, in two studies with a total of 706 participants from The Netherlands and Canada, a 12-item scale for individual differences in intrasexual competition was developed that was sex neutral, and that had a high degree of crossnational equivalence. In The Netherlands, sociosexuality, sex drive and social comparison orientation were independently related to intrasexual competition. In Canada, intrasexual competition was strongly, and independently of the Big Five, related to social comparison orientation, but only among women. There was no effect of birth order, but sibling rivalry did correlate with intrasexual competition. Among men, intrasexual competition was more strongly, and differently, related to the Big Five than among women. Among women, intrasexual competition was predicted by a lack of agreeableness, and among men by a high level of neuroticism and extraversion.