The Parkinson's disease (PD) substantia nigra is characterized by the presence of Lewy bodies containing fibrillar alpha-synuclein. Early-onset PD has been linked to two point mutations in the gene that encodes alpha-synuclein, suggesting that disease may arise from accelerated fibrillization. However, the identity of the pathogenic species and its relationship to the alpha-synuclein fibril has not been elucidated. In this in vitro study, the rates of disappearance of monomeric alpha-synuclein and appearance of fibrillar alpha-synuclein were compared for the wild-type (WT) and two mutant proteins, as well as equimolar mixtures that may model the heterozygous PD patients. Whereas one of the mutant proteins (A53T) and an equimolar mixture of A53T and WT fibrillized more rapidly than WT alpha-synuclein, the other (A30P) and the corresponding equimolar mixture with WT fibrillized more slowly. However, under conditions that ultimately produced fibrils, the A30P monomer was consumed at a comparable rate or slightly more rapidly than the WT monomer, whereas A53T was consumed even more rapidly. The difference between these trends suggested the existence of nonfibrillar alpha-synuclein oligomers, some of which were separated from fibrillar and monomeric alpha-synuclein by sedimentation followed by gel-filtration chromatography. Spheres (range of heights: 2-6 nm), chains of spheres (protofibrils), and rings resembling circularized protofibrils (height: ca. 4 nm) were distinguished from fibrils (height: ca. 8 nm) by atomic force microscopy. Importantly, drug candidates that inhibit alpha-synuclein fibrillization but do not block its oligomerization could mimic the A30P mutation and thus may accelerate disease progression.