Gravity studies show that the subsurface part of the Long Valley caldera is a concident steep-sided depression filled with porous epielastic and volcanic materials to a depth of as much as 3 km. The depression contains two major basins, a larger and deeper one making up much of the eastern part and a somewhat smaller, more shallow one to the west; a positive feature underlying the central part of the depression separates the two basin areas. The east side of this feature is linear in plan and coincides with the extension of the Hilton Creek fault which is mapped within and beyond the south edge of the caldera. The indicated relief on the postulated subsurface Hilton Creek fault together with difference in depth of the eastern and western basinal areas indicates that the eastern basin is downdropped in relation to the western one. Gentle gravity gradients outside the caldera but sloping towards it are interpreted as evidence of a low-density mass located below the caldera fill. We conclude that it is probably related to the magma source. Aeromagentic data indicate that a northwest-trending belt of metasedimentary rocks on the south flank of Long Valley may extend into the caldera proper and form much of the bedrock floor of the western part of the caldera. A magnetic low of shallow source in the hot spring region in the southwest is thought to be caused by hydrothermal alteration of the ferrimagnetic minerals in the underlying rocks. A broad positive magnetic anomaly near the center of the caldera may be caused by a thick section of magnetic volcanic flow lying east of the projected Hilton Creek fault and underlying much of the eastern basin.