A pathological feature of Parkinson's disease is the presence of Lewy bodies within selectively vulnerable neurons. These are ubiquitinated cytoplasmic inclusions containing α-synuclein, an abundant protein normally associated with presynaptic terminals. Point mutations in the α-synuclein gene (A30P and A53T), as well as triplication of the wild-type (WT) locus, have been linked to autosomal dominant Parkinson's. How these alterations might contribute to disease progression is unclear. Using the genetically tractable yeast Saccharomyces cerevisiae as a model system, we find that both the WT and the A53T isoforms of α-synuclein initially localize to the plasma membrane, to which they are delivered via the classical secretory pathway. In contrast, the A30P mutant protein disperses within the cytoplasm and does not associate with the plasma membrane, and its intracellular distribution is unaffected by mutations in the secretory pathway. When their expression is elevated, WT and A53T, but not A30P, are toxic to cells. At moderate levels of expression, WT and A53T induce the cellular stress (heat-shock) response and are toxic to cells bearing mutations in the 20S proteasome. Our results reveal a link between plasma membrane targeting of α-synuclein and its toxicity in yeast and suggest a role for the quality control (QC) system in the cell's effort to deal with this natively unfolded protein.