Ultrathin-film polymeric transistors with controlled in-plane chain orientation have been fabricated based on Langmuir-Blodgett technique by cospreading liquid crystal molecule with regioregular poly(3-hexylthiophene). The mobilities parallel to the chain direction reached 0.001-0.01 cm(2)/V s for 1 to 5 monolayer thick transistors. Mobility ratio was about 2 for the parallel and perpendicular directions. Electron spin resonance (ESR) signals of the field-induced polarons exhibited clear in-plane anisotropies due to unpaired pi-electrons. The anisotropic ESR spectra together with the optical dichroism determine the detailed molecular orientations in the channel of such ultrathin transistors.