Through ecological and epidemiological studies, the distribution ofCoccidioides immitis has been shown to be restricted to the Americas. The saprophytic nature of the organism and its tolerance for high soil salinity in climates of high temperature and low rainfall is known. The distribution ofC. immitis in nature seems to coincide with the Lower Sonoran Life Zone though not altogether inclusive. Epidemiological studies and case reports have extended the apparent range of the organism; however, almost all soil isolates in Arizona and California have been from areas with the physical and biotic parameters of this zone. The repeated isolation ofC. immitis from the soil has been from small, well defined areas; in some instances the positive area has been no more than 24 square feet. The isolations from soil have been randomed and associated either with animal burrows around creosote bushes, Amerind habitation sites, or fossil beds.C. immitis has been observed to grow in the soil if naturally infected soil is simply enriched with sterile water. The ease with which it can be grown is paradoxical to its naturally restricted distribution to soils in endemic areas. Microbial inhibition has been reported for a limited number of microorganisms from endemic soil, however, no general antagonism in all soils has been seen. Observations of colonial and microscopic variations in both naturally occurring isolates and laboratory produced auxotrophs have extended the textbook description of the organism to a broader species complex. More is known about the disease coccidioidomycosis than about the organism that causes it.