Allometry, or size-related variation, is shown to be an important factor in the bifaces of two separate assemblages from the Acheulean site of Kariandusi in Kenya. Such variation has functional and possibly stylistic implications. The paper gives a review of the archaeology of Kariandusi, and then investigates the question of size and shape variation in bifaces. The Upper Sites at Kariandusi, first investigated by L. S. B. Leakey, have yielded many obsidian bifaces which can be dated to approximately 0.7–1.0 myr ago. Material from the Lower Site, including many lava bifaces, is judged to be stratigraphically younger but probably in the same time range. We show by making comparisons with the neighbouring Acheulean sites of Kilombe and the Kapthurin Formation that allometry is present in biface assemblages at least 0.5 myr different in date; that similar principles of allometry operate in all the assemblages; and that where there are differences of allometric pattern, within-site variation is sometimes greater than variation between distant sites. We conclude that the size of a biface at least partly determines the shape in which it was made, and that sites in the time range ofHomo erectus and earlyHomo sapiens show surprisingly similar allometric patterns.