In September 1982, the U.S. Geological Survey recorded a 125-km-long refraction profile through the Lassen Peak area of the Cascade Range. This profile extended northwestward to the Klamath Mountains and southeastward to the Sierra Nevada foothills. Interpretation of the refraction data determined the upper-crustal structure in this volcanic region of northern California. Near-surface velocities (1.5--5.55 km/s) are consistent with a thickness of 2--4 km for the Cascade volcanic rocks. Between 0.5 km above and 1.8 km below sea level, velocities are about 4.7--5.55 km/s. The highest velocities are modeled below the Devils Kitchen-Terminal Geyser area, possibly indicating hydrothermal alteration of the volcanic rocks. The absence of low velocities (less than about 3--4.5 km/s) deeper than 1 km above sea level suggests that the Cretaceous sedimentary rocks exposed west of Lassen Peak do not underlie the volcano. Southeast of Lassen Peak, at 1.8 km below sea level, a 0.3-km-thick layer with a velocity of 6.1 km/s overlies a lower-velocity (5.7--5.8 km/s) 2.9-km-thick layer. Because these modeled velocities do not change across the boundary between the Sierra Nevada foothills and the Cascade Range, Sierran basement could continue under the Cascade volcanic rocks as far northwest as Lassen Peak. The velocity model does not indicate the presence of a magma chamber larger than 3 km in diameter within the top 8 km of the crust. ¿American Geophysical Union 1987