Nine strains were studied of a new haploid species of the genus Debaryomyces Lodder&Kreger-van Rij that had been isolated from exudates of mesquite (Prosopis juliflora) trees in southern Arizona and from Drosophila carbonaria that breeds in these exudates. Their physiological characteristics, life cycle, and nuclear DNA base composition (approx. 37?5 mol% G+C) led to their original classification as Debaryomyces hansenii (Zopf) Kreger-van Rij. However, the two taxa are, at most, distantly related based on DNA reassociation values that indicated low base sequence complementarity. The two taxa also have distinctly different electrophoretic karyotypes. Debaryomyces hansenii has two varieties, D. hansenii var. hansenii and D. hansenii var. fabryi. Debaryomyces prosopidis can be differentiated phenotypically from both varieties by lack of growth on cellobiose after 2 weeks incubation and from the variety hansenii by a higher maximum temperature for growth. The type strain of Debaryomyces prosopidis sp. nov. is strain UCD-FST 84?100T(= DBVPG 7010T= CBS 8450T= ATCC 201611T).