Charge transport in the ribbon phase of poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (PBTTT)-one of the most highly ordered, chain-extended crystalline microstructures available in a conjugated polymer semiconductor-is studied. Ribbon-phase PBTTT has previously been found not to exhibit high carrier mobilities, but it is shown here that field-effect mobilities depend strongly on the device architecture and active interface. When devices are constructed such that the ribbon-phase films are in contact with either a polymer gate dielectric or an SiO2 gate dielectric modified by a hydrophobic, self-assembled monolayer, high mobilities of up to 0.4 cm(2) V-1 s(-1) can be achieved, which is comparable to those observed previously in terrace-phase PBTTT. In uniaxially aligned, zone-cast films of ribbon-phase PBTTT the mobility anisotropy is measured for transport both parallel and perpendicular to the polymer chain direction. The mobility anisotropy is relatively small, with the mobility along the polymer chain direction being higher by a factor of 3-5, consistent with the grain size encountered in the two transport directions.