A detailed seismic refraction study was undertaken of the rifted continental margin southeast of the Grand Banks of Newfoundland, along a preexisting deep multichannel seismic reflection profile. The primary objectives were to determine the topography of Moho across the margin, to detect any evidence for crustal underplating, and to determine the position of the continent-ocean boundary. Data were obtained from nine ocean bottom seismometers deployed along the profile, with an array of six 1000-inch3 air guns as the source. Two additional refraction lines were shot parallel to structure over the continental slope. The resultant velocity model is well constrained and consistent with the observed gravity. The crust thins abruptly beneath the continental slope to a major structural transition identified as the continent-ocean boundary. There is no significant magmatic underplating of the thinned continental crust. Seaward of the transition, the seismic velocity structure is characterized by a 50-km-wide zone of intermediate velocity (7.2-7.6 km/s). Beyond this is a thin (3-5 km) curst with low velocity (4.5-5 km/s) and high gradient, over mantle with a velocity of 7.7 km/s increasing to 8.0 km/s. The zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust, suggesting little or no magmatic activity during and immediately after rifting. This interpretation is consistent with results from adjacent and conjugate margins. ¿ American Geophysical Union 1994