A new class of nanoparticle filaments and tree-like aggregates is assembled by the influence of an electric field during the synthesis of metallic, intermetallic, silicon, and carbon nanoparticles from the vapor phase. Enormous electrostatic aggregation due to dipole forces is observed between the nanoparticles to form chain filaments, and between the chain filaments to form tree-like fibers. The filaments and tree-like fibers can grow to lengths exceeding several centimeters. The filaments display stretch and contraction properties depending on the strength of the applied field.