The Neoproterozoic Port Askaig Formation contains widespread pyrite within many diamictite beds, across Scotland and Ireland. The quantity of pyrite is anomalous for coarse-grained rocks, especially in rocks deposited at a time when seawater contained low sulphate levels due to a continental ice cover which inhibited weathering. Sulphur isotope compositions evolve from lightest values (down to -3.1‰) at the base of the formation to highly positive compositions in the overlying Bonahaven Dolomite (mean +44.8‰). This trend is consistent with progressive utilization of available sulphate by closed system microbial sulphate reduction. Together with records from other contemporary diamictite successions, there emerges a picture of global microbial activity during Neoproterozoic ‘Snowball Earth’ glaciation.