Three inches (7.6 cm) of differential heave has caused severe structural deformation of a lightly loaded building founded directly on drained black shale bedrock in Ottawa, Canada. Portions of the building founded on shale below the water table have not heaved.The heave is attributed to oxidation of disseminated iron sulfide in the shale by autotrophic bacteria to produce secondary hydrous sulfates of greater volume. Autotrophic bacteria of the Ferrobacillus and Thiobacillus ferrooxidan types, which cause this type of oxidation in warm, drained, humid environments, were confirmed by laboratory cultures to exist in the rock waters.The potential importance of heave and sulfate attack on future underground concrete structures in areas of Ottawa underlain by black shale is discussed.