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Mammals with more rapid and agile locomotion have larger semicircular canals relative to body mass than species that move more slowly. Measurements of semicircular canals in extant mammals with known locomotor behaviours can provide a basis for testing hypotheses about locomotion in fossil primates that is independent of postcranial remains, and a means of reconstructing locomotor behaviour in species known only from cranial material. Semicircular canal radii were measured using ultra high resolution X-ray CT data for 9 stem primates (“plesiadapiforms”; n=11), 7 adapoids (n=12), 4 omomyoids (n=5), and the possible omomyoid Rooneyia viejaensis (n=1). These were compared with a modern sample (210 species including 91 primates) with known locomotor behaviours. The predicted locomotor agilities for extinct primates generally follow expectations based on known postcrania for those taxa …