ABSTRACT Candida albicans is an important human pathogen that displays a remarkable ability to detect changes in its environment and to respond appropriately by changing its cell morphology and physiology. Serum- and amino acid-based media are known to induce filamentous growth in this organism. However, the mechanism by which amino acids induce filamentation is not yet known. Here, we describe the identification and characterization of the primary amino acid sensor of C. albicans , Csy1. We show that Csy1p plays an important role in amino acid sensing and filamentation. Loss of Csy1p results in a lack of amino acid-mediated activation of amino acid transport and a lack of induction of transcription of specific amino acid permease genes. Furthermore, a csy1 Δ/ csy1 Δ strain, lacking Csy1p, is defective in filamentation and displays altered colony morphology in serum- and amino acid-based media. These data provide the first evidence that C. albicans utilizes the amino acid sensor Csy1p to probe its environment, coordinate its nutritional requirements, and determine its morphological state.