This paper reports that the opaque and white phenotypes of Candida albicans constitute a true high-frequency reversible transition system. The rDNA restriction fragment and orthogonal field alternating gel electrophoresis profiles of opaque and white phenotypes are indistinguishable, and a genetic marker introduced into a white strain is present in all opaque derivatives of this strain. Opaque and white derivatives appear markedly different on a bismuth indicator medium and differ in a number of other respects. We have used bismuth medium to examine the spontaneous and temperature-induced frequencies of transition from opaque to white. The temperature-induced transition from opaque to white does not occur when opaque cells are held in water.