Sample: pyrite, largely oxidized to gypsum. The pyrite is cementing sandstone, interpreted to be an injected sandstone.Objective: as part of a larger survey of pyrite in injected sandstones, the pyrite was analysed for sulphur isotopic composition, to determine if microbial activity was involved in its precipitation. The analysis was done using standard methods at the Scottish Universities Environmental Research Centre, East Kilbride, Glasgow.Interpretation: the very light composition, compared to Oligocene seawater (~+20 per mil), is strong evidence for precipitation by microbial sulphate reduction. The fractionation between the two of over 30 per mil exceeds the 20 per mil maximum likely from non-biological processes.