Six sites were drilled in the South Atlantic during the Leg 73 cruise of the Deep Sea Drilling Project (DSDP). Hydraulic piston coring at five of the six sites obtained a nearly complete sequence of undisturbed Cenozoic samples. The magnetostratigraphy at those sites was investigated by close sampling representing time intervals of about 104 yr. Most of the Cenozoic nannofossil and many of the foraminiferal zonal boundaries were accurately determined and magnetostratigraphically calibrated at those five Leg 73 boreholes. Their numerical ages have been computed assuming a linear spreading rate and a radiometric age of 66.5 m.y. for the Cretaceous-Tertiary boundary. Alternative magnetostratigraphic ages were obtained with the adoption of a 63.5-m.y. age for the Cenozoic. Our data confirm previous determinations of the Pleistocene-Pliocene boundary at 1.8 (1.7) m.y. B.P. and of the Pliocene-Miocene boundary at 5.1 (5.0) m.y. B.P. The Miocene-Oligocene boundary is placed within chron C-6 C, with a magnetostratigraphic age of 23.8–24.0 (22.7–22.9) m.y. The Oligocene-Eocene age is also very precisely located within chron C-13 R, with a magnetostratigraphic age of 37.1–37.2 (35.5–35.6) m.y. The Eocene-Paleocene boundary should be located within an uncored interval of chron C-24, with a magnetostratigraphic age of 59.0 (55.4) ± 0.2 m.y. A general accord of the magnetostratigraphic ages from Leg 73 sites and the radiometric ages published in the literature for the various zonal boundaries validates the assumption of a linear sea-floor–spreading rate during the Cenozoic.