We have designed and synthesized novel semiconducting polymers by introducing naphtho[1,2-b:5,6-b?]dithiophene (NDT) into the polythiophene backbone. These polymers, which have a highly π-extended heteroarene unit, achieved mobilities (>0.5 cm2 V?1 s?1) that are among the highest recorded to date for semiconducting polymers and most probably result from the highly ordered thin-film structures with crystalline close π stacking. It is noteworthy that the choice of isomeric heteroarenes in the unit can dramatically change the physical and electronic structures and hence the OFET performance of the semiconducting polymers, even though the two isomers possess similar electronic structures; interestingly, this contrasts with the trend in small-molecule systems. We believe that these findings will give new insight into the design of new organic semiconducting materials and that the present polymers are promising materials for printable electronics.