Organic field-effect transistors (FETs) based on epitaxially grown crystals of a thiophene/phenylene co-oligomer (see Figure and cover) are described. The FETs exhibit good operation characteristics, and the epitaxial needle-like crystals display good charge transport properties along the needle axis. The maximum hole mobility of 0.66 cm(2) V-1 s(-1) is close to that of vapor phase grown oligothiophene single crystals.