In the title compound, C(19)H(30)N(2)O, the fused ring system is essentially planar, the maximum deviation from the mean plane being 0.013 (2) Å for the N atom bearing the dodecyl chain. The 1-dodecyl group is almost perpendicular to the 1H-benzo[d]imidazol-2(3H)-one plane as indicated by the dihedral angle of 82.9 (2)°between planes through the fused ring system and the first three C atoms of the chain. The C-C-C-C torsion angles (about ±179°) of the dodecyl group indicate an anti-periplanar conformation. In the crystal, inversion dimers are formed by pairs of N-H⋯O hydrogen bonds.