Single-channel and multichannel marine seismic data off the coast of West Antarctica collected during two Nathaniel B. Palmer cruises (NP92-8 and NP96-2) in the vicinity of 65 ¿S to 71 ¿S, 220 ¿E to 250 ¿E, reveal a NNW trending graben. We interpret this graben to be part of the paleodivergent plate boundary between the Marie Byrd Land and Bellingshausen plates. This graben coincides with a -520 nT magnetic anomaly to the NNW and a -720 nT anomaly to the SSE, as well as a 20 mGal negative gravity anomaly. Seismic profiles subparallel to the graben (22 km/Ma half-spreading rate) reveal greater seafloor roughness to the NE, where seafloor spreading was slower, than to the SW (27 km/Ma half-spreading rate). These data allow the position of the Marie Byrd Land-Bellingshausen plate boundary to be constrained more precisely than has previously been possible, with a trend of N17 ¿W from 68.52 ¿S, 233.65 ¿E to 68.41 ¿S, 233.56 ¿E. The sediment-filled graben has normal separation of sedimentary layers varying from 740¿30 m to 580¿20 m imaged in seafloor of age A33y (74 Ma). ¿ 1999 American Geophysical Union