Coccidioides posadasii sp. nov., formerly known as non-California Coccidioides immitis. Coccidiodes immitis is a dimorphic pathogenic fungus found in southewectern United States, Mexico, Central and South America. It is characteristically found inhabiting the arid, sandy soils of the Lower Sonoran Life Zone. C. posadasii represent a divergent, genetically recombining monophyletic clade. C. posadasii can be distinguished from C. immitis by numerous DNA polymorphisms. In this paper, they used phylogenetic analyses of microsatellite loci to identify isolates as C. immitis or C. posadasii. A cursory analysis of data might lead to the conclusion that genetic isolation between C. immitis and C. posadasii was not absolute, due to the occurence of alleles that are shared between two taxa. Thus, the recognition if the two species has enable us to show that C. posadasii grows slowly on media containing high salt concentration. The range in growth overlaps between the two species, showing that although the difference in growth rates is significants, this phenotype is not diagnostic. The presence of differences in the amino acid composition of proteins between the two species show that other phenotypic differences may exist.