Thermally dimorphic fungal pathogens grow in the soil in a multicellular, filamentous form and transition to a unicellular, pathogenic cell type within mammalian hosts. Temperature is the only known environmental trigger for this morphogenetic switch, but Gilmore et al. now show that the monosaccharide N-acetylglucosamine (GlcNAc), which is ubiquitous in the environment, is a robust inducer of the unicellular-to-multicellular transition in both Histoplasma capsulatum and Blastomyces dermatitidis. Transcriptional profiling revealed that GlcNAc-induced morphogenesis in H. capsulatum depends on the expression of two GlcNAc transporters, Ngt1 and Ngt2. Moreover, Ngt1 and Ngt2 were also important for the efficiency of this transition even in the absence of exogenous GlcNAc, which suggests that they function as sensors of endogenous GlcNAc to regulate morphogenesis in response to temperature.