A ground magnetic survey has further delineated the previously known intense (1000 nT) negative anomaly in the centre of the Lake St. Martin structure, Manitoba. Shocked gneiss in the central uplift has a strong reverse remanent magnetization whose direction is similar to that found after alternating field demagnetization of melt rock specimens from within the impact structure. Shocked gneiss is interpreted as the source of the central negative anomaly. The direction of the remanence is consistent with the Permo-Triassic age for the impact event. The magnetization in the shocked gneiss is interpreted as a chemical remanent magnetization carried by hematite formed during an extensive alteration of mafic silicates, occurring immediately after the impact.