Mating between natural a/a and alpha/alpha strains of Candida albicans requires that cells first switch from the white to opaque phase phenotype. However, because cells expressing the opaque phase phenotype are induced to switch back to the white phase phenotype at physiological temperature (37 degrees C) and because opaque phase cells are highly efficient at colonizing skin, we tested whether skin, which is several degrees lower than physiological temperature, is conducive to mating. Using a model in which a mixture of a/a and alpha/alpha cells are incubated for 24 h under a cotton patch on the hairless skin of newborn mice and using scanning electron microscopy to visualize cells on skin, it was demonstrated that skin facilitates mating. In some regions of the skin, 40% of all cells had fused. All of the stages of mating observed in vitro were observed in vivo. However, some unique morphological characteristics of mating on skin were observed and are attributable to parent cell immobilization on the skin. In control experiments on glass, plastic, and silicone elastomer surfaces at 32 degrees C, cells either failed to fuse or did so at an extremely low frequency, suggesting that unique features of the skin surface other than reduced temperature also facilitate fusion.