Deposits from the banks of rivers in the Peruvian Amazon basin have already produced a gomphothere mastodont below an Ar/Ar date of 9.01 Ma, making it significantly older than the closing of the Panamanian land bridge at 3.5 Ma. Magnetostratigraphic studies on the overlying Madre de Dios Formation were undertaken to more precisely date these pre-3.5 Ma mammal-bearing deposits. Samples were taken as either oriented blocks, or in Pyrex glass vials, and treated with AF and thermal demagnetization. Most sites yielded a stable remanence held largely in magnetite which was pointed north and up (normal in the southern hemisphere) or south and down (reversed in the southern hemisphere). A total of 16 polarity zones were obtained in the 65-m-thick Cerro Colorado section. Based on Ar/Ar dates of 9.01 Ma at the base and 3.12 Ma at the top, we correlate these magnetozones with Chrons C4Ar to C2An (3.0-9.5 Ma). The 45-m-thick R?o de las Piedras section has only 5 polarity zones for the same time duration, so there must be considerable condensation of section and/or unconformities which have wiped out polarity intervals. Nonetheless, these results show that the Madre de Dios Formation spans most of the late Miocene and early Pliocene (at least 3 to 10 Ma), and that the fossils of mastodonts, peccaries and camelids found below it are older than at least 9 Ma. Consequently, the idea that all these North American mammal groups arrived when the Panamanian land bridge formed at 3.5 Ma needs to be reassessed.