The thiophene-quinoxaline donor-acceptor conjugated copolymer poly [(thiophene-2,5-diyl-alt- (2,3-diheptylquinoxaline-5.8-diyl)] (PTHQx) was explored as a semiconductor in thin-film organic field-effect transistors (OFETs). A hole mobility of 3.6 x 10(-3) cm(2) center dot V-1 center dot s(-1) and an on/off current ratio of 6 x 10(5) were observed in p-channel OFETs made from spin-coated PTHQx thin films. The electronic structures of PTHQx and a related thiophene-thienopyrazine donor-acceptor copolymer were calculated by density functional theory. Atomic force microscopy of PTHQx thin films showed a polycrystalline grain morphology that varied with the substrate.