Receiver function analysis of 14 teleseismic events recorded by 6 temporary PASSCAL broadband stations within the Altiplano--Puna volcanic complex (APVC) shows a consistent ~2 s negative-polarity P-to-S conversion for all stations for all available azimuths. Forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of ~19 km, with a Vs<0.5 km/s and a thickness of 750--810 m. We interpret the characteristics of the low-velocity zone (low Vs, areal extent, and flatness) to be consistent with a still-like magma body. On the basis of additional data from the German ANCORP experiment, the Altiplano--Puna magma body appears to underlie much of the APVC, and it may therefore be the largest known active continental crustal magma body. ¿ 1999 American Geophysical Union