Results of the LITHOPROBE transect provide additional corroboration of the impact model for the origin of the Sudbury Structure. High resolution seismic reflection data indicate that the Sudbury Igneous Complex (SIC) is indeed continuous under the Sudbury Basin. There is no indication of a feeder dike system or a previously assumed ''hidden mafic mass.'' All these observations strongly support the interpretation of the SIC as a coherent impact melt sheet. In addition, the seismic, and to a lesser extent magnetic and gravity, results point to a highly asymmetric structure to the SIC at depth, with significant tectonic disruption in the South Range. The present non-circularity of the SIC is caused by post-impact ductile deformation and thrusting due to northwest directed compression. Decompressing this strain pattern leads to an original diameter of the SIC between 60 and 80 km, which, in consequence, would extend the original diameter of the final crater up to a maximum of 280 km. ¿ American Geophysical Union 1994