A systematically regiocontrolled synthesis of poly(3-alkylthiophenes) (P3AT) mediated by Rieke zinc is reported. Rieke zinc undergoes oxidative addition to 2,5-dibromo-3-alkylthiophene or 2-bromo-5-iodo-3-alkylthiophene regioselectively to afford 2-bromo-5-(bromozincio)-3-alkylthiophene (2) or 2-bromo-5-(iodozincio)-3-alkylthiophene (10). The intermediate 2 or 10 can be polymerized catalytically to a series of regiospecific poly(3-alkylthiophenes) using different catalysts. The regioregularity of the polymer chain is solely controlled by the structure of the catalyst. An almost completely regioregular head-to-tail (HT) P3AT (4) is obtained by using Ni(DPPE)Cl-2 ([1,2-bis-(diphenylphosphino)ethane]nickel(II) chloride). Use of Pd(DPPE)Cl-2 leads to a reduction in the regioregularity (70:30 HT/HH), while using Ni(PPh(3))(4) also leads to a much reduced regioregular P3AT (63:35 HT/HH). A totally regiorandom (50:50 HT/HH) P3AT (5) is afforded by using Pd(PPh(3))(4). The poly(3-butylthiophene) 4a is a 97% HT regioregular polymer. Other poly(3-alkylthiophenes) (alkyl = hexyl (4b), octyl (4c), decyl (4d), dodecyl (4e), and tetradecyl (4f)) are regioregular P3ATs with the HT linkage larger than 98.5% based on NMR analysis. Electronic absorption, X-ray diffraction, and crossed polarizing micrograph studies show that the cast films of the regioregular P3ATs (4) are self-organized, crystalline, flexible, and bronze-colored films with a metallic luster, while that of the regiorandom P3ATs (5) are amorphous and orange-colored films. The regioregular P3ATs exhibit a small bandgap (1.7 eV) which is 0.4 eV lower than that of regiorandom P3ATs (2.1 eV). Regioregular HT P3ATs have considerably improved electroconductivity and other physical properties over regiorandom P3ATs.