Arboreal locomotion is characteristic for many taxa within the mammalian superclade Euarchontoglires, which includes, e.g., primates, and rodents. Early fossils of Euarchontoglires share various morphological similarities with many extant arboreal mammals (small body size, claws on all digits, limited prehensility), suggesting arboreal locomotion to be the ancestral condition of this superclade. The locomotion of extant species is often studied to understand the necessary biomechanical adaptations of early arboreal ancestors. However, studies commonly focus on symmetrical gaits typically used at lower running speeds or when confronted with compliant or very narrow substrates. Fast locomotion exploiting asymmetrical gaits is rarely considered. The small squirrel Tamiops swinhoei (Scuiromorpha) is a representative species of this type of locomotion. We analyzed its locomotion at different running speeds while confronting it with different inclinations and substrates (flat runway vs. narrow pole). We simultaneously recorded high-Hz videos and substrate reaction forces during 450 trials. The squirrels almost exclusively used asymmetric gaits. Differences between locomotion on the narrow pole and on the flat runway were interpreted as typical adjustments to arboreal locomotion also found for symmetrical gaits. However, the differences themselves strongly depended on inclination and running speed, hinting at the important role of biomechanical fine-tuning during fast arboreal locomotion. Our findings complement previous studies in developing an understanding of the locomotor behavior of the ancestors of Euarchontoglires.