We have found evidence for a dikelike structure in the middle of the Albuquerque-Belen basin of the Rio Grande rift from our interpretation of two-station Rayleigh wave amplitude and phase velocity data. Two-dimensional finite element modeling suggests that the structure is about 200 m thick, extends vertically from a depth of about 2 1/2 km to at least 20 Km, and contains material characterized by a shear wave velocity of about 1/2 km/s. Simple physical arguments suggest that the length of the structure may be at least 28 km long. Nearby high heat flow values, historical swarmlike seismicity, recent (0.14 Ma) volcanism, and an extensive, sill-like magma body at midcrustal depths all suggest that the structure is a magma-filled dike. Source characteristics and seismometer response limit our reliable data to the frequency band of 0.08 Hz to 0.250 Hz; thus we cannot tell whether the dike is solely an upper crustal feature or whether it extends through the crust and (in)to the upper mantle. ¿ American Geophysical Union 1987