We report the synthesis, characterization, solution-phase assembly of nanowires, field-effect charge transport, and photovoltaic properties of regioregular poly(3-pentylthiophene) (P3PT), the first regioregular poly(3-alkylthiophene) with an odd-numbered alkyl side chain length to be so investigated. Two samples of P3PT with weight-average molecular weights of 61800 and 77 000 and 1,4-1.5 polydispersity index have comparable solubility and processability in organic solvents as regioregular poly(3-hexylthiophene) (P3HT) but have a higher melting transition at 259 degrees C. X-ray diffraction of P3PT films revealed a lamellar structure with an interlayer d(100) spacing of 1.51 nm and a pi-stacking d(010) spacing of 0.374 nm, both smaller than in P3HT. Crystalline nanowires of 16-17 nm width and aspect ratios as high as 465 were assembled from P3PT solution. Field-effect transistors fabricated from P3PT thin films showed a mobility of holes of up to 0.10 cm(2)/(V s), and the carrier mobility increased with molecular weight. Bulk heterojunction solar cells based on P3PT/fullerene (PC71BM) blend thin Films had a power conversion efficiency of 3.70% under 100 mW/cm(2) AM1.5 solar illumination in air and a maximum external quantum efficiency of 69%. Similarly illuminated solar cells based on nanocomposites of P3PT nanowires/PC71BM had a 3.33% power conversion efficiency. These results demonstrate that P3PT is an attractive polymer semiconductor suitable for applications in thin-film and nanowire-based field-effect transistors and bulk heterojunction solar cells.