We report on the electrical conductance of nanofibers of regioregular poly(3-hexylthiophene) (RRP3HT) as a function of gate-induced charge. Nanofibers of RRP3HT were deposited onto SiO2/Si substrates by casting from dilute p-xylene solutions. An analysis of the nanofibers by atomic force microscopy revealed fiber lengths of 0.2-5 mum, heights of 3-7 nm, and widths of approximately 15 nm. A field effect transistor geometry was used to probe the conductance of webs of nanofibers and single nanofibers; in these measurements, gold electrodes served as source and drain contacts, and the doped SiO2/Si substrate served as the gate. Temperature-dependent transport studies on webs of nanofibers revealed an activation energy of 108 meV at a gate-induced hole density of 3.8 x 10(12) charges/cm(2). Pretreating SiO2 with a hydrophobic hexamethyldisilazane (HMDS) layer reduced the activation energy to 65 meV at the same charge density. The turn-on gate voltage on treated and untreated substrates increased in magnitude with decreasing temperature. Conductance measurements on single nanofibers on HMDS-treated SiO2 yielded hole mobilities as high as 0.06 cm(2)/Vs with on/off current ratios greater than 10(3). (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2674-2680, 2003