regation of alpha-synuclein (aS) into toxic fibrils is a pathogenic hallmark of Parkinson disease (PD). Studies have focused largely on residues 71–82, yet most early-onset mutations are located between residues 46 and 53. A semirationally designed 209,952-member library based entirely on this region was constructed, containing all wild-type residues and changes associated with early-onset PD. Cell survival screening using a protein-fragment competition assay followed by growth competition was used to isolate a 10-residue peptide antagonist that potently inhibits aS aggregation and associated toxicity at a 1:1 stoichiometry. Dose dependence was verified using continuous growth measurements and ThT cytotoxicity studies, with atomic force microscopy and circular dichroism on the same samples highlighting a random-coil structure and no oligomers. A new region of aS for inhibitor targeting has been highlighted, together with a new approach of using a semirational design and intracellular screening. The peptides can then be used as candidates for modification in drugs capable of slowing or even preventing the onset of PD.