The Baja BC interpretation remains controversial due to concerns about the reliability of the paleomagnetic data and apparent conflicts with long-held views of the tectonic evolution of western North America. Paleobotanical estimates of mean-annual temperature can be used to infer the paleolatitude of suspect terrenes relative to stable North America. The Winthrop Formation, an Albian-Cenomanian fluvial unit in the Methow/Tyaughton basin of northern Washington State, provides an exceptional terrestrial section to test the Baja BC hypothesis because it lies within Baja BC and predates the proposed 85-50 Ma translation event. Our preliminary investigations of the Winthrop Formation have yielded a diverse flora of 32 species of sphenophytes, pteridophytes, cycadophytes, gymnosperms, and angiosperms, many of which were previously unknown in the region. This angiosperm-dominated flora is comparable to the Potomac Group middle-upper subzone II-B and subzone II-C (Hickey and Doyle, 1977) indicating a middle Albian to early Cenomanian age. Twenty-three species of dicotyledonous angiosperm leaves from the Winthrop and correlative Pasayten Formation in British Columbia were analyzed using leaf shapes to estimate mean-annual temperature (Wilf, 1997). We estimate a mean-annual temperature of ~25.5° ± 2.5°C (± 1 SE), indicating a tropical climate, comparable to present-day Miami (Wolfe, 1979). This result is supported by the paleogeographic affinity and high diversity of the flora. In comparison, nearby mid-Cretaceous floras in the Northern Rocky Mountains indicate a warm-temperate climate (Crabtree, 1987), comparable to present-day St. Louis (Wolfe, 1979). Thus, our Winthrop/Pasayten estimates appear to support a low-latitude provenance for these floras. Further work is in progress to improve the precision of these temperature estimates, and their correlation to paleolatitude.