Abstract: Many fungi undergo a developmental transition from a unicellular yeast form to an invasive filamentous form in response to environmental cues. Here we describe a quorum signaling pathway that links environmental sensing to morphogenesis in Saccharomyces cerevisiae. Saccharomyces cells secrete aromatic alcohols that stimulate morphogenesis by inducing the expression of FLO11 through a Tpk2p-dependent mechanism. Mutants defective in synthesis of these alcohols show reduced filamentous growth, which is partially suppressed by the addition of these aromatic alcohols. The production of these auto signaling alcohols is regulated by nitrogen: High ammonia restricts it by repressing the expression of their biosynthetic pathway, whereas nitrogen-poor conditions activate it. Moreover, the production of these aromatic alcohols is controlled by cell density and subjected to positive feedback regulation, which requires the transcription factor Aro80p. These interactions define a quorum-sensing circuit that allows Saccharomyces to respond to both cell density and the nutritional state of the environment. These same autoregulatory molecules do not evoke the morphological switch in Candida albicans, suggesting that these molecular signals are species-specific.