Gravity measurements at 821 stations in an area of about 4000 square miles of the southern Cascade Range in and around Lassen Volcanic National Park, California, revealed a large gravity low of amplitude 70 mgals, with steepest gradients of 8 mgals per mile, and an area of about 2000 square miles. Interpretation of this gravity low leads to the conclusion that it is caused by a large mass of material of low density. This mass has a volume of about 15,000 km3 if the density contrast between it and the enclosing crustal rocks is 0.2 g per cm3. The mass excess of the Cascade Range in the Lassen Park area is in approximate isostatic equilibrium with the buried low-density mass, and the upper surface of this low-density mass must be in the upper part of the earth's crust. The low-density mass could be made up of: (1) a batholith of silicic rock; (2) a thick accumulation of low-density sedimentary deposits; (3) a low-density mass caused by thermal expansion of crustal rocks by heat from igneous activity; (4) a volcano-tectonic depression filled with volcanic material of low average density, or some combination of these.