Przewalski's gazelle (Procapra przewalskii) and the Tibetan gazelle (P. picticaudata) are endemic, closely related, and endangered ungulates of the Qinghai-Tibetan Plateau. The 2 species often occur in mixed-species groups in the upper Buha River of the Plateau. We studied the composition and size of their mixed-species groups over 2 years to determine whether such groups aggregate by chance and to determine a posteriori potential costs and benefits associated with the formation of mixed-species groups. Sex composition and size distribution were similar in single-species groups for both species. Given that population density also was similar for these species, we expected that mixed-species groups that formed by chance would consist of an equal mix of the 2 species. This was true in male and in mixed-sex groups; however, the proportion of female groups composed of Przewalski's gazelles was much larger than expected. In addition, mixed-species groups in winter never included males of both species. The results suggest that these 2 gazelle species do not associate randomly. Mixed-species groups were larger than single-species all-female, all-male, and mixed-sex groups, suggesting that individuals in larger groups may benefit from a reduction in predation risk. The occurrence of mixed-sex, mixed-species groups may increase the risk of crossbreeding and represent a cost to the formation of mixed-species groups in these two gazelle species.