Horizontally stratified magnetic zones are observed throughout the Pliocene-Pleistocene valley fill of the upper San Pedro Valley, Arizona. Within one detailed 150-m stratigraphic section (Curtis Ranch), twelve superposed magnetic polarity zones are identified spanning the time from the upper Gilbert Epoch through the Brunhes Epoch. These magnetic polarity zones extend over a 64-km stretch of the valley and serve as convenient chronostratigraphic units. Based on the position of mammalian fossils within the chronostratigraphic zones, we have defined four mammal datum planes for the upper San Pedro Valley, including the first appearance of Lepus sp. at 1.9 m.y. Lepus sp. marks the first occurrence of a definitive Irvingtonian land mammal in the San Pedro Valley and thus places the local boundary of the Irvingtonian Land Mammal Age shortly below the Olduvai event. This position equates approximately with the Pliocene-Pleistocene boundary of the pelagic marine record.