Four magnetic anomaly profiles across the Marquesas swell were obtained on the Crossgrain expedition of the R/V Thomas Washington in April 1987. The data cover a poorly dated region of the Pacific between the Galapagos and Marquesas fracture zones. Magnetic lineations trending approximately 350¿ in the vicinity of the swell were identified as anomalies 20 through 29, differing from identifications proposed by previous workers. Our identifications imply that the seafloor beneath the Marquesas Islands formed at a half-spreading rate of approximately 40--45 mm/yr during Paleocene and early Eocene time (65 to 50 Ma). The offset across the Marquesas fracture zone apparently increased from approximately 700 to 850 km between the times of anomalies 21 and 13. ¿ American Geophysical Union 1988