As a result of the CANNIKIN nuclear explosion, the magnetic field several kilometers from the epicenter appears to have been permanently altered. Within 30 sec after detonation, a proton magnetometer 3 km away recorded a 9-gamma step increase in total magnetic field. Continuous difference recordings between the station at a distance of 3 km from the epicenter and one at 9 km showed that a 7.0-gamma average increase was maintained between these locations for 8 days after the shot. Along a -km traverse centered across a portion of a fault 1.6 km from ground zero, postshot-minus-preshot magnetic field difference readings decreased semi-sinusoidally from a high value of +13 gammas in the shot-contained block to a low value of −11 gammas in the distal block. Within 15 m to either side of the fault trace, a 15-gamma wedge-shaped magnetic low was observed. The magnetic effects can be reasonably interpreted as being caused by either shot-produced residual stresses or shot-caused alterations of remanent magnetization.