The mobility and contact resistance of transistors based on regioregular poly(3-hexylthiophene) (P3HT) with Ti/Pt electrodes were investigated as a function of the molecular weight (MW) of P3HT. For an increase in MW from 5.5 to 11 kDa, the mobility increased from 0.04 to 0.16 cm2 V-1 s-1, whereas the contact resistance decreased from 1.7 to 0.6 M Ω. Further increases in MW yielded an apparent saturation in both the mobility and the contact resistance. A nanofibrilar morphology was observed where the width of the nanofibrils increases with MW. A qualitative model based on polymer chain folding is proposed to explain the electrical results.