Helium isotope ratios (3He/4He) in Lassen Park and Yellowstone Park volcanic gases show large 3He enrichments relative to atmospheric and crustal helium indicating the presence of a dominant mantle-helium component. The ratios in Lassen helium are 8 times atmospheric in acid hot spring gases, and about 3 times atmospheric in the gas phase in near-neutral high-temperature waters; the acid-spring helium is thus isotopically similar to helium in island-arc and other continental-margin orogenic areas. At Yellowstone, however, the isotopic ratio in helium from the Mud Volcano area, in the eastern part of Yellowstone caldera, is 15.6 times atmospheric, similar to the ratio in helium from Kilauea and Iceland. These high ratios, 15 times atmospheric or greater, are probably distinctively associated with deep-mantle plumes under hot-spots, while ratios about 10 times atmospheric characterize mantle helium in basalt glasses at the crests of mid-ocean rises. The presence of Kilauea-type helium at Yellowstone indicates that at least in certain areas the continental crust is essentially transparent to mantle volatiles.