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The human fungal pathogen Candida albicans has at least two types of phenotypicswitch systems for growth transitions: whiteto opaquecell transitions and yeast to hyphaltransitions.Opaque cells have historically not been known to undergo filamentation under standardfilament-inducing conditions.Here we find that Bcrl and its downstream regulators Cup9,Nrgland Czfl andthe cAMP-signaling pathway control opaquecell filamentation in C.albicans.Wehave shown that deletion of BCRI,CZIP9,NRGI andCZFI results in opaque cell filamentationunder standard culture conditions.Disruption of BCRI and CZFI in white cells has no obviouseffect on hyphal growth,suggesting that Bcrl and Czfl are opaque specificregulators offilamentation under the condirions tested.Moreover,inactivation of the cAMP-signaling pathwayor disruption of its downstream transcriptional regulators,FL08 and EFGI,strikingly attenuatesfilamentation in opaque cells of the bcrl/bcrlmutant. Deletion of HGCl,adownstream gene ofthe cAMP-signaling pathwayencoding Gl cyclin-related protein,completely blocks opaque cellfilamentation induced byinactivation of BCRI.