Isotopic and paleontological proxies indicate warming of 4-8 degrees C in mid-high latitudes over a period of ~10 kyr following the Paleocene/Eocene boundary. The warming lasted ~150 kyr, during which temperatures returned to warm Paleogene background conditions. This Paleocene-Eocene Thermal Maximum (PETM) is coincident with a major immigration of mammals into North America and Europe, and change in marine faunas as well. Continental floral change at the PETM is less dramatic, and includes a small number of first appearances, and shifts in the relative abundances of taxa that are already present. Until recently the timing of climatic and floral change at the PETM could not be compared because of an absence of plant fossils from this brief interval. Four lines of evidence indicate that the lower Wasatch Fm. on Chalk Butte in the Powder River Basin of Wyoming records the PETM. The rocks are lithologically similar to the intensely red, variegated paleosol horizons of PETM sequences in the nearby Bighorn Basin, the carbon isotope values of pedogenic carbonate nodules are light, mammalian fossils from the Chalk Butte area record the Wa0 fauna characteristic of the PETM, and the pollen floras through the section change from typically Paleocene to typically Eocene composition. Unlike PETM sections in the Bighorn Basin, at Chalk Butte pollen is preserved in two samples from within the PETM. These samples show that characteristic Eocene composition was not acquired until at least 40 m above the first excursion values. Assuming average sedimentation rates of 0.3-0.5 m/kyr the floral shift lagged peak PETM warming by approximately 80 kyr. We present three hypotheses for the lag between warming and floral change: 1) latitudinal gradients of floral composition were low in the Paleogene so that even large shifts in temperature did not cause major phytogeographic boundaries to cross the mid-latitude site sampled here; 2) migration of thermophilic plants into North America across high-latitude corridors was not possible during the PETM because polar night conditions prevented them from establishing; and 3) plant migration during the PETM was slow because successful long-distance dispersal and establishment was improbable in the widespread forest vegetation