Submersible magnetic profiles over the MIR and ALVIN relict hydrothermal zones of the TAG hydrothermal field on the Mid-Atlantic Ridge at 26 ¿N show magnetic anomalies that are consistent with the presence of reduced crustal magnetization. These magnetic anomalies are similar to the anomaly found associated with the TAG active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones. Magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems. ¿ American Geophysical Union 1996