The field-effect mobility of holes in regioregular poly(3-alkylthiophene)s was determined for a series of 5 alkyl chain lengths from C4 (n-butyl) to C12 (n-dodecyl). Contrary to a previous report, a non-monotonic dependence of field-effect mobility on alkyl chain length was found. The average hole mobility varied from 1.2 × 10−3 cm2/Vs in poly(3-butylthiophene) and 1 × 10−2 cm2/Vs in poly(3-hexylthiophene) to 2.4 × 10−5 cm2/Vs for poly(3-dodecylthiophene). We believe that the hexyl side chain is optimum for charge transport because of better self-organization in poly(3-hexylthiophene) compared to other polymers in the series. The present results provide an important structure–carrier mobility relationship for the regioregular poly(3-alkylthiophene)s which are of wide interest for thin film transistors and photovoltaic cells.