Alpha-synuclein (αS) fibrils have been recognized as the main component that make up Lewy bodies, which are a hallmark for Parkinson's disease (PD). The misfolding and aggregation of αS is implicated as a causative agent of PD, however the correlation between αS aggregation and the ultimate progression of PD is not yet clear. Oligomers, short proto-fibrils and mature amyloid fibrils of αS have all been shown to be cytotoxic, and these αS species are also able to spread from neuron-to-neuron and induce endogenous αS aggregation. The inhibition of αS fibril formation and reduction of the toxic species produced is an active target for therapeutic intervention. A homologous protein, beta-synuclein (βS), has been found to delay αS fibril formation in vitro and reduce the formation of Lewy bodies in vivo. However, the influence of βS on the cytotoxicity of αS aggregate species is unknown. Here, we show that co-incubation of αS and βS monomers forms an amyloid fibril which appears to have the same fibril core structure as the αS fibril, but exhibits a reduced cytotoxicity, reduced seeding ability, and greater thermostability. In addition, as a dynamic equilibrium is reached, oligomers and proto-fibrils that shed from mature co-incubated fibrils show reduced toxicity relative to oligomers shed from mature αS fibrils. These results suggest that βS directly modulates the fibril shedding and seeding processes to act as a neuroprotective modifier of αS toxicity.