We have investigated the functional role of the non-helical end domains of vimentin on its assembly properties using truncated Xenopus and human recombinant proteins. Removal of the amino-terminal “head” domain yielded a molecule that did not assemble into 10 nm filaments but remained in a soluble oligomeric particle form with a sedimentation coefficient considerably smaller than that of wild-type vimentin (Vim(wt)). In contrast, removal of the carboxy-terminal “tail” domain had no obvious effect on the sedimentation characteristics. In particular, sedimentation equilibrium analysis under low ionic strength conditions yielded oligomeric particle species ofM_r135,000 to 360,000, indistinguishable from those obtained with Vim(wt). When induced to form filaments from this state by rapid dilution into filament forming buffer, Vim(wt) and Vim(ΔT) protein generated similar viscosity profiles. However, as determined by scanning transmission electron microscopy, under these conditions Vim(ΔT) formed filaments of heterogeneous diameter, corresponding to various distinct mass-per-length (MPL) values: whereas Vim(wt) yielded MPL values peaking between 40 and 45 kDa/nm, Vim(ΔT) filaments produced histograms which could be fitted by three Gaussian curves peaking between 37 and 131 kDa/nm. In contrast, when dialyzed against, instead of being rapidly diluted into, filament forming buffer, Vim(ΔT) gave histograms with one major peak at about 54 kDa/nm. The MPL heterogeneity observed for Vim(ΔT) was already evident at the earliest stages of assembly. For example, ten seconds after initiation, “unit-length” filament segments (58 to 63 nm) were formed with both wt and ΔT proteins, but the diameters were considerably larger for Vim(ΔT) compared to Vim(wt) (20(±3) nmversus16(±3)nm), indicating a distinct role of the carboxy-terminal tail domain in the width control during unit-length filament formation. Despite this difference both Vim(ΔT) and Vim(wt) filaments appeared to grow stepwise in a modular fashion from such unit-length filament segments. This suggests that assembly occurred by a principally similar mechanism involving the end-on-fusion or annealing of unit-length filaments.