We present new geochronologic magnetostratigraphy, fission-track and 40Ar/39Ar radioisotopic dates, biostratigraphy)bdata constraining the age of three separate sequences and a composite section from Guanajuato, Mexico. Those data make this one of the most complete and precisely age-calibrated sequences in North America spanning the Hemphillian/Blancan North American Land Mammal “Age” (NALMA) boundary interval, and the data further constrain the timing and pattern of the Great American Biotic Interchange (GABI). In total, 196 samples (77 sites) were used to construct the magnetic polarity stratigraphies, with eight fission-track analyses and four new 40Ar/39Ar radioisotopic dates. The sections sampled are possibly latest Miocene to Late Pliocene in age, spanning about 2.7 m.yr. (5.5–2.7 Ma), from Chrons C3n.4n or C3r to C2An.1n. The radioisotopic dates and magnetostratigraphic correlations indicate the Hemphillian/Blancan NALMA boundary at Guanajuato in central Mexico is ?4.7–4.8 Ma, and it probably lies within Chron C3n.2r, consistent with most prior age estimates and correlations. Our analyses indicate that a variety of “invaders” (taxa with South American ancestry, including Megalonyx, Glossotherium, and Plaina) clearly were present by at least 4.7–4.8 Ma and therefore much earlier in lower latitude Middle America than in more temperate parts of North America. Others (e.g., Neochoerus, Glyptotherium) were present shortly thereafter but still much earlier than in higher latitudes. Thus, the first appearances of these five immigrant taxa can no longer be used to define the beginning of the late Blancan. This timing significantly predates the earliest documented major influxes of North American forms into South America, is at least 1.5–2 m.yr. earlier than the beginning of the GABI previously recorded in North America (beginning of the Blancan [Bl 2], 2.7 Ma), and suggests faunal provinciality and more staggered arrival and dispersal of South American immigrants than indicated in temperate sequences.