Seismic refraction, reflection, and gravity data obtained across the Peru continental margin and Nazca plate at lat 9°S permit a detailed determination of crustal structure. The western portion of the continental shelf basement consists of a faulted outer continental shelf high of Paleozoic or older rocks. It is divided into a deeper western section of velocity 5.0 km/sec. The combined structure forms a basin of depth 2.5 to 3.0 km which contains Tertiary sediments of velocity 1.6 to 3.0 km/sec. The 3 km thick, 4.55 to 5.15 km/sec basement of the eastern shelf shoals shoreward. Together, this basement and the eastern section of the outer continental shelf high form a synclinal basin overlain by Tertiary sediment which have a maximum thickness of 1.8 km and a velocity range of 1.7 to 2.55 km/sec. The gravity model shows a large block of 3.0 g/cm3 lower crustal material emplaced within the upper crustal region beneath the eastern portion of the continental shelf. Refraction data indicate a continental slope basement of velocity 5.0 km/sec overlying a slope core material with an interface velocity of 5.6 km/sec. The sedimentary layers of the slope consist of an uppermost layer of slumped sediment with an assumed velocity of 1.7 to 2 km/sec that overlie an acoustic basement of 2.25 to 3.6 km/sec. The high velocities (and densities) of the slope basement suggest the presence of oceanic crustal material overlain by indurated oceanic and continental sediments. This slope mélange may have formed during the initiation of subduction from imbricate thrusting of upper layers of oceanic crust. A ridgelike structure within the trench advances the seismic arrival times of deeper refractions and supports the suggestion that it is trust-faulted oceanic crust which has been uplifted relative to the trench floor. The model of the descending Nazca plate consists of a 4 km thick upper layer of velocity 5.55 km/sec and a thinner (2.5 km) but faster (7.5 km/sec) lower layer that overlies a Moho of velocity 8.2 km/sec. The gravity model indicates that the plate has a dip of 5° beneath the continental slope and shelf. West of the trench, the lower crustal layers rise upward, which may represent upward flexure of the oceanic plate due to compressive forces resulting from the subduction process. The upper crustal layers of the 120 km long oceanic plate portion consist of a thin, 1.7 km/sec sedimentary layer overlying a 5.0 to 5.2 km/sec upper layer. Immediately beneath these layers, a 5.6 to 5.7 km/sec lower layer becomes more shallow to the east within 60 km of the trench, and a deeper 6.0 to 6.3 km/sec layer thickens to the east. The lower crustal model consists of a 7.4 to 7.5 km/sec high velocity layer that varies in thickness from 2.5 km to 4.0 km. The 8.2 km/sec Moho interface varies not more than ±0.5 km from a modeled depth of 10.5 km.