The dielectric constant of flagellin proteins in flagellar bacterial filaments ∼10-20 nm in diameter is measured using scanning dielectric microscopy. We obtained for two different bacterial species (Shewanella oneidensis MR-1 and Pseudomonas aeruginosa PAO1) similar relative dielectric constant values εSo = 4.3 ± 0.6 and εPa = 4.5 ± 0.7, respectively, despite their different structure and amino acid sequence. The present results show the applicability of scanning dielectric microscopy to nanoscale filamentous protein complexes and to general 3D macromolecular protein geometries, thus opening new avenues to study the relationship between the dielectric response and protein structure and function.