α-synuclein is a major component of intraneuronal protein aggregates constituting a distinctive feature of Parkinson disease. To date, fluorescence imaging of dynamic processes leading to such amyloid deposits in living cells has not been feasible. To address this need, we generated a recombinant α-synuclein (α-synuclein-C4) bearing a tetracysteine target for fluorogenic biarsenical compounds. The biophysical, biochemical and aggregation properties of α-synuclein-C4 matched those of the wild-type protein in vitro and in living cells. We observed aggregation of α-synuclein-C4 transfected or microinjected into cells, particularly under oxidative stress conditions. Fluorescence resonance energy transfer (FRET) between FlAsH and ReAsH confirmed the close association of fibrillized α-synuclein-C4 molecules. α-synuclein-C4 offers the means for directly probing amyloid formation and interactions of α-synuclein with other proteins in living cells, the response to cellular stress and screening drugs for Parkinson disease.