We have studied the carrier transport in poly(2,5-bis(3-tetradecylthiophen- 2-yl)thieno[3,2-b]thiophene) field-effect transistors (FETs) at very high field-induced carrier densities (1015 cm-2) using a polymer electrolyte as gate and gate dielectric. At room temperature, we find high current densities, 2 × 106 A/cm2, and high metallic conductivities, 104 S/cm, in the FET channel; at 4.2 K, the current density is sustained at 107 A/cm2. Thus, metallic conductivity persists to low temperatures. The carrier mobility in these devices is ≈3.5 cm2.V-1·s-1 at 297 K, comparable with that found in fully crystalline organic devices. © 2006 by The National Academy of Sciences of the USA.