Charge carriers induced in field-effect transistors based on a uniaxially aligned polythiophene polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b] thiophene) (PBTTT), are investigated by electrooptical charge modulation spectroscopy (CMS). We used a zone-casting deposition method for uniaxial alignment of the nanoribbon crystalline domains of the polymer and examined the optical anisotropy of neutral and charge induced absorptions in FETs. We find that the charge-induced optical absorptions of the polaronic carriers at the interface exhibit a similar degree of uniaxial anisotropy as the absorption spectrum of the neutral polymer bulk suggesting similar degree of polymer alignment at the interface compared with the bulk. We observe subtle differences in the spectral shape of the polaron absorption polarized along and perpendicular to the polymer chain direction. We also detect an additional charge-induced absorption peak appearing at high charge carrier concentrations, which is similar to the charge-induced absorption that is typical for chemically doped films. These observations provide important insight into the interplay between polaron transport and polymer microstructure. © 2013 American Chemical Society.