A deep geological repository in stable plutonic rock is one of the approaches being considered for the long-term management of used nuclear fuel in Canada. This study examines the interaction of cesium, which contains 135Cs, a long-lived radionuclide that is a component of nuclear waste, and adsorption of Cs onto a variety of clay minerals representing the materials that are likely to be used as engineered barriers in such a repository. Clay minerals are also likely to be associated with fracture zones in the crystalline rock hosting the repository. The Lac du Bonnet batholith in southeastern Manitoba is representative of plutonic rocks of the Canadian Shield. The batholith is sparsely fractured and of a large areal extent. The effect of a bacterial consortium from the Lac du Bonnet batholith on Cs adsorption was also evaluated. Ion exchange experiments indicate that clay minerals adsorb cesium rapidly at various conditions of temperature and pressure. At room temperature and pressure, montmorillonite, a 2:1 layer clay mineral, adsorbed more Cs than all the other clay minerals. Vermiculite and illite, though 2:1 layer clay minerals, adsorbed much less Cs than montmorillonite. Bentonite (Wyoming bentonite and Kunigel bentonite), a multimineralic clay, adsorbed more Cs than vermiculite and illite but less than montmorillonite. Kaolinite, a 1:1 layer clay mineral, adsorbed very small amounts of Cs. Generally,2:1 layer clays adsorbed more Cs than 1:1 layer clay because they have more available sites for Cs adsorption...