Coccidioides posadasii is a pathogenic fungus that, along with Coccidioides immitis, is the causative agent of coccidioidomycosis ("California valley fever") in humans. It resides in the soil in certain parts of the Southwestern United States, northern Mexico, and certain other areas in the Americas.
C. posadasii and C. immitis are morphologically identical but genetically and epidemiologically distinct. C. posadasii was identified as a separate species than C. immitis in 2002. The two species can be distinguished only by genetic analysis and different rates of growth in the presence of high salt concentrations (C. posadasii grows more slowly). It is also found outside the San Joaquin Valley, unlike its relative; while C. immitis is geographically limited to the valley, C. posadasii can also be found in the desert southwest of Mexico and South America.