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Three cycles of successive trap cultures were utilized to determine the taxonomic structure of arbuscular fungi associated with mesquite (Prosopis spp.) from three locations in the Sonoran desertscrub biotic community and one location in the Chihuahuan desertscrub biotic community. Unlike the low species richness found in previous studies of some arid habitats, seven to nine species were recovered from each sampling site. This number is comparable to that found in many other plant communities. Seventy-five percent of the species found after three culture cycles were not detected in the first trap cultures, suggesting that a high proportion of arbuscular fungi in arid habitats may be nonsporulating in the field. Low colonization levels may account for the absence of sporulation, but ecological factors such as moisture limitations also may be involved. Although a total of 10 different species was detected, generic richness was limited, with 9 of these species in Glomus and 1 in Entrophospora. As long as measurement of species richness is based on occurrence of sporulation, successive trap cultures provide a means of detecting nonsporulating mycorrhizal colonizers in arid and other habitats.