ABSTRACT The early to middle Pleistocene Colorado River Delta deposits exposed in the upper Gulf of California, Sonora, México, are host to a diverse paleo-fauna and paleo-flora (El Golfo local paleobiota) of Irvingtonian Land Mammal Age (Calabrian Stage). The fossiliferous exposures are found in badlands developed in fluvio-deltaic sediments that have been mildly deformed during late Pleistocene doming along the Cerro Prieto fault. The El Golfo Project is part of the resource inventory for the Upper Gulf of California and Colorado River Delta Biosphere Reserve. Through joint efforts by Arizona Western College, the La Brea Tar Pits and Museum, and the Centro de Geociencias, Universidad Nacional Autónoma de México, about 50% of the region has been prospected and mapped. To date, more than 13,500 mapped vertebrate fossil localities are documented, including important microvertebrate sites. New mammalian, avian, fish, and flora taxa have been recently added to a growing paleobiota list that now numbers more than 145 taxa. The preserved paleobiota suggests the existence of three ecologic communities: freshwater aquatic, shrub and brush woodland, and savannah-like grassland. Some of the fauna are presently endemic to geographic areas farther south, suggesting a more tropical to subtropical climate on the Colorado River Delta during this part of the Irvingtonian.