Because Candida dubliniensis is closely related to Candida albicans, we tested whether it underwent white-opaque switching and mating and whether white-opaque switching depended on MTL homozygosity and mating depended on switching, as they do in C. albicans. We also tested whether C. dubliniensis could mate with C. albicans. Sequencing revealed that the MTLα locus of C. dubliniensis was highly similar to that of C. albicans. Hybridization with the MTLa1, MTLa2, MTLα1, and MTLα2 open reading frames of C. albicans further revealed that, as in C. albicans, natural strains of C. dubliniensis exist as a/α, a/a, and α/α, but the proportion of MTL homozygotes is 33%, 10 times the frequency of natural C. albicans strains. C. dubliniensis underwent white-opaque switching, and, as in C. albicans, the switching was dependent on MTL homozygosis. C. dubliniensis a/a and α/α cells also mated, and, as in C. albicans, mating was dependent on a switch from white to opaque. However, white-opaque switching occurred at unusually high frequencies, opaque cell growth was frequently aberrant, and white-opaque switching in many strains was camouflaged by an additional switching system. Mating of C. dubliniensis was far less frequent in suspension cultures, due to the absence of mating-dependent clumping. Mating did occur, however, at higher frequencies on agar or on the skin of newborn mice. The increases in MTL homozygosity, the increase in switching frequencies, the decrease in the quality of switching, and the decrease in mating efficiency all reflected a general deterioration in the regulation of developmental processes, very probably due to the very high frequency of recombination and genomic reorganization characteristic of C. dubliniensis. Finally, interspecies mating readily occurred between opaque C. dubliniensis and C. albicans strains of opposite mating type in suspension, on agar, and on mouse skin. Remarkably, the efficiency of interspecies mating was higher than intraspecies C. dubliniensis mating, and interspecies karyogamy occurred readily with apparently the same sequence of nuclear migration, fusion, and division steps observed during intraspecies C. albicans and C. dubliniensis mating and Saccharomyces cerevisiae mating.