This study is aimed at defining the boundary and internal structure of the Novarupta vent area, the source of the 1912 Katmai eruption. A geodetic grid consisting of 145 stations was occupied for gravity and magnetic measurements and an aeromagnetic survey was flown at 1100 m above sea level. The combined gravity and magnetic data indicate that there are probably two intrusive bodies within the vent, not just one at Novarupta, as was previously thought. The outline of the overall vent at the surface is well defined by the aeromagnetic survey, and the vent appears to dip inwards indicating that it flares from depth. This is in contrast to the piston-like collapse thought to have taken place at Mt. Katmai in 1912. ¿American Geophysical Union 1991