Posttranslational modifications of proteins play critical roles in the control of cellular differentiation, development, and environmental adaptation. In particular, the covalent attachment of the small ubiquitin-like modifier, SUMO, to target proteins (sumoylation) regulates cell cycle progression, transcription, nucleocytoplasmic transport, and stress responses. Here we combine proteomic, molecular, and cellular approaches to examine the roles of sumoylation in the major fungal pathogen of humans, Candida albicans. Using an N-terminally FLAG-tagged SUMO, 31 sumoylated proteins were identified in C. albicans with roles in stress responses (e.g., Hsp60, Hsp70 family members, Hsp104), the cytoskeleton and polarized growth (e.g., Tub1, Cct7, Mlc1), secretion, and endocytosis (e.g., Lsp1, Sec24, Sec7). The output from this proteomic screen was entirely consistent with the phenotypes of C. albicans mutants in which the single SUMO-encoding locus (SMT3) was inactivated or down-regulated. C. albicans smt3/smt3 cells displayed defects in growth, morphology, cell separation, nuclear segregation, and chitin deposition, suggesting important roles for sumoylation in cell cycle control. Smt3/smt3 cells also displayed sensitivity to thermal, oxidative, and cell wall stresses as well as to the antifungal drug caspofungin. Mutation of consensus sumoylation sites in Hsp60 and Hsp104 affected the resistance of C. albicans to thermal stress. Furthermore, signaling via the cell integrity pathway was defective in C. albicans smt3/smt3 cells. These observations provide mechanistic explanations for many of the observed phenotypic effects of Smt3 inactivation upon C. albicans growth and environmental adaptation. Clearly sumoylation plays key roles in fundamental cellular processes that underpin the pathogenicity of this medically important fungus.