Aim: The fate of micro-organisms in the bentonite clay surrounding high-level radioactive waste (HLW)-containing copper canisters in a future Swedish underground (500 m) repository were investigated. Methods and Results: Laboratory experiments were designed in which the mixing of various bacterial species with swelling bentonite was studied. A clear trend of fewer cultivable bacteria at depth was seen in the clay. This trend was consistent as the incubation time was increased from 8 h to 28 weeks. Sulphate-reducing bacteria were found to be active, reducing sulphate at the lowest density studied, 1.5 g cm(-3), but sulphate reduction activity ceased at higher densities. Conclusions: The number of viable micro-organisms in an HLW repository bentonite clay buffer will decrease rapidly during swelling and very few viable cells will be present at full compaction. Significance and Impact of the Study: Sulphate-reducing bacteria will most probably not be able to induce corrosion of HLW-containing copper canisters.