Aggregation of a peptide, alpha synuclein, into beta-sheet structured amyloid has been believed to be implicated in the pathology of Parkinson's disease. A considerable amount of data has identified the soluble alpha synuclein oligomers as potentially significant toxic agents. However, the possibility that the alpha synuclein fibrils may be associated with neurotoxicity cannot be ruled out, since fibrillar aggregates can serve as a pool of soluble intermediate species through a dynamic exchange with monomers or oligomers. Development of compounds controlling alpha synuclein aggregation would be greatly beneficial in not only reducing toxic effects but also developing accurate functional correlations between an aggregation state and toxicity. We have designed a peptidyl aggregation modulator composed of divalent “binding domain” and “conformational domain”. The binding domain contains an amino acid sequence responsible for binding toward alpha synuclein. The conformational domain was added to prevent self assembly of binding domains. A peptidyl modulator was tested in the recently developed Escherichia coli expression system, where the effects of aggregation modulators on alpha synuclein amyloidogenesis can be examined in vivo. Our results indicated that the addition of the designed peptidyl modulator caused conversion of insoluble alpha synuclein aggregates to small soluble species in vivo. Examination of alpha synuclein aggregation in vitro in the presence of the peptidyl modulator is underway.