Mutations in the DJ-1 gene have been implicated in the autosomal recessive early onset parkinsonism. DJ-1 is a soluble dimeric protein with critical roles in response to oxidative stress and in neuronal maintenance. However, several lines of evidence suggest the existence of a nonfunctional aggregated form of DJ-1 in the brain of patients with some neurodegenerative diseases. Here, we show that inorganic phosphate, an important anion that exhibits elevated levels in patients with Parkinson disease, transforms DJ-1 into filamentous aggregates. According to the 2.4-A crystal structure, DJ-1 dimers are linearly stacked through P(i)-mediated interactions to form protofilaments, which are then bundled into a filamentous assembly.