Candida albicans and related species switch frequently and reversibly between a number of general phenotypes usually discriminated by colony morphology and in some cases by cellular morphology. Switching has been demonstrated to affect a number of physiologic and architectural characteristics of single cells including most of the putative virulence factors of C.albicans. In the past few years, we have cloned several genes regulated by switching in the white-opaque transition of C.albicans strain WO-1. Two of the genes, PEP1 and Op4, are transcribed only in the opaque phase, and one of the genes, Wh11, is transcribed only in the white phase. These coordinately regulated genes are unlinked in the genome and do not undergo sequence reorganization in switching. With the identification of a cis-acting regulatory sequence in the five-prime flanking sequence of Wh11, we now believe that phase-specific genes are regulated by transacting factors and that these factors may be coded for or under the direct regulation of a single master regulatory gene at which site the basic switch event occurs.