Conductive ultra thin films were fabricated from mixed monolayers containing stearic acid and structurally homogeneous poly(3-hexylthiophene), which had almost 100% head-to-tail couplings with a M(w) of 8100 (M(w)/M(n) = 1.5) as estimated by H-1 NMR, C-13 NMR, and gel permeation chromatography. The mixed monolayers were found to be stable at the air-water interface and could be readily deposited onto solid substrates as Y-type films by the vertical lifting method. These Langmuir-Blodgett (LB) films exhibited well-defined layered structures as determined by UV-Vis absorption and X-ray diffraction measurements. The conductivity of the mixed LB films doped with NOPF6 was in the range of 50-100 S cm(-1) and was 50- to 100-fold higher than that of the mixed LB films using poly(3-hexylthiophene) synthesized by the usual chemical oxidation with FeCl3.