Nitrogen starvation is one of the signals that induce Candida albicans, the major fungal pathogen of humans, to switch from yeast to filamentous growth. In response to nitrogen starvation, C. albicans expresses the MEP1 and MEP2 genes, which encode two ammonium permeases that enable growth when limiting concentrations of ammonium are the only available nitrogen source. In addition to its role as an ammonium transporter, Mep2p, but not Mep1p, also has a central function in the induction of filamentous growth on a solid surface under limiting nitrogen conditions. When ammonium is absent or present at low concentrations, Mep2p activates both the Cph1p-dependent mitogen-activated protein (MAP) kinase pathway and the cAMP-dependent signalling pathway in a Ras1p-dependent fashion via its C-terminal cytoplasmic tail, which is essential for signalling but dispensable for ammonium transport. In contrast, under ammonium-replete conditions that require transporter-mediated uptake Mep2p is engaged in ammonium transport and signalling is blocked such that C. albicans continues to grow in the budding yeast form. Mep2p is a less efficient ammonium transporter than Mep1p and is expressed at much higher levels, a distinguishing feature that is important for its signalling function. At sufficiently high concentrations, ammonium represses filamentous growth even when the signalling pathways are artificially activated. Therefore, C. albicans has established a regulatory circuit in which a preferred nitrogen source, ammonium, also serves as an inhibitor of morphogenesis that is taken up into the cell by the same transporter that mediates the induction of filamentous growth in response to nitrogen starvation.