Self-assembled crystalline nanowires (NWs) of poly(3-butylthiophene) (P3BT), having an average width of 11.8 nm and aspect ratios of similar to 330-850, and their nanocomposites with insulating polystyrene (PS) were found to have a high field-effect mobility of holes and a high dc conductivity. The intra-nanowire carrier mobility in the P3BT-NW/PS nanocomposites can be as high as 0.2 cm(2) V-1 s(-1), which is significantly enhanced compared to films of pure P3BT NWs. Nanocomposites of P3BT NWs with polystyrene were found to support high carrier densities (10(17)-10(18) cm(-3)) with high mobility of holes, facilitating the development of high performance field-effect transistors. The observed high carrier mobility and high conductivity in the nanocomposites were explained by the rather low percolation threshold (0.5 wt % P3BT NWs).