Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are characterized by the presence of filamentous inclusions in nerve cells. These filaments are amyloid fibrils that are made of the protein alpha-synuclein, which is genetically linked to rare cases of PD and DLB. beta-Synuclein, which shares 60% identity with alpha-synuclein, is not found in the inclusions. Furthermore, while recombinant alpha-synuclein readily assembles into amyloid fibrils, beta-synuclein fails to do so. It has been suggested that this may be due to the lack in beta-synuclein of a hydrophobic region that spans residues 73-83 of alpha-synuclein. Here, fibril assembly of recombinant human alpha-synuclein, alpha-synuclein deletion mutants, beta-synuclein and beta/alpha-synuclein chimeras was assayed quantitatively by thioflavin T fluorescence and semi-quantitatively by transmission electron microscopy. Deletion of residues 73-83 from alpha-synuclein did not abolish filament formation. Furthermore, a chimera of beta-synuclein with alpha-synuclein(73-83) inserted was significantly less fibrillogenic than wild-type alpha-synuclein. These findings, together with results obtained using a number of recombinant synucleins, showed a correlation between fibrillogenesis and mean beta-strand propensity, hydrophilicity and charge of the amino acid sequences. The combination of these simple physicochemical properties with a previously described calculation of beta-strand contiguity allowed us to design mutations that changed the fibrillogenic propensity of alpha-synuclein in predictable ways.