Proton transport properties of a partially protonated poly(aspartic acid)/sodium polyaspartate (P-Asp) were investigated. A remarkable enhancement of proton conductivity has been achieved in the thin film. Proton conductivity of 60-nm-thick thin film prepared on MgO(100) substrate was 3.4 X 10(-3) S cm(-1) at 298 K. The electrical conductivity of the oriented thin film was 1 order of magnitude higher than the bulk specimen, and the activation energies for the proton conductivity were 0.34 eV for the oriented thin film and 0.65 eV for the pelletized sample, respectively. This enhancement of the proton transport is attributable to the highly oriented structure on MgO(100) substrate. This result proposes great potential for a new strategy to produce a highly proton-conductive material using the concept of an oriented thin film structure without strong acid groups.