Coccidioides posadasii sp. nov., formerly known as non-California (non-CA) Coccidioides immitis, is described. Phylogenetic analyses using single nucleotide polymorphisms, genes, and microsatellites show that C. posadasii represents a divergent, genetically recombining monophyletic clade. Coccidioides posadasii can be distinguished from C. immitis by numerous DNA polymorphisms, and we show how either of two microsatellite loci may be used as diagnostic markers for this species. Growth experiments show that C. posadasii has significantly slower growth rates on high-salt media when compared with C. immitis, suggesting that other phenotypic characters may exist.