Teleseismic earthquakes recorded within the ISSA (Integrated Seismological Experiment in the Southern Andes) temporary seismic experiment in the Southern Andes between 36° and 40°S latitude have been used to construct receiver function images of the crust and upper mantle. The oceanic Moho of the subducted Nazca plate is observed down to a depth of ∼100 km, corresponding well with the Wadati-Benioff zone seismicity and wide-angle seismic reflections. Beneath the volcanic arc, the slab begins to be invisible with P-to-S converted waves, implying the completion of the gabbroeclogite transformation in the oceanic crust at that depth. The continental Moho has been imaged at depth of ∼40 km beneath the main cordillera and shallows toward the eastern end of the profile beneath the Neuquén Basin to ∼35 km depth. Beneath the Loncopué graben, the Moho is locally uplifted to 30 km depth, possibly resulting from the backarc spreading beginning in the Pliocene-Pleistocene. An anomalously high Poisson's ratio beneath the volcanic arc may indicate partial melting in the upper-plate crust.