Two seismic refraction profiles crossing the Long Valley caldera in approximately east and north directions indicate that the crystalline basement with P wave velocities of 6.0¿0.4 km/s has been downdropped by 2.5--3 km across normal faults along the north and northwest sides of the caldera and 1--2 km along the south and east sides. Basement depths beneath the caldera floor range from between 3 and 4 km in the north and east sections to be about 2 km in the central and south sections. Relief on the basement within the caldera suggests that the caldron block was partially disrupted during collapse, although a steplike offset in the eastern part of the basement may be due in part to precollapse displacement along the Hilton Creek fault. The distribution of P wave velocities in the caldera fill suggests that the Glass Mountain rhyolite and Bishop tuff have velocities of 4.0--4.4 km/s and the postcollapse rhyolite, rhyodacite, and basalt flows have velocities of 2.7--3.4 km/s. Domelike relief on the 4.0- to 4.4-km/s horizon indicates that postcollapse resurgence elevated the west central part of the caldera by about 1 km. Evidence for the roof of the magma chamber is contained in later arrivals tentatively identified as reflections from a low-velocity horizon at a depth of 7--8 km. Evidence for anomalous scattering or absorption properties assoicated with the region of shallow hydrothermal alteration and hot spring activity is contained in relative attenuation of high frequencies in a guided wave propagating through this region.