Temperature and gate voltage dependent transport measurements on single grain boundaries in the organic semiconductor sexithiophene (6T) are described. Isolated grain boundaries are formed by vacuum deposition of pairs of 6T grains between Au electrodes 1.5-2.0 μm apart on SiO2/Si substrates; grain boundary formation is monitored using atomic force microscopy. The Si substrate serves as the gate electrode. We show from the activation energy, threshold voltage, and field effect resistance of the grain boundary junction that carrier transport is limited by the grain boundary. The activation energy of room temperature transport is of the order of 100 meV at carrier densities of ∼1018cm-3 and decreases with increasing carrier concentration (gate voltage). We also observe that longer grain boundaries with smaller misorientation angles result in larger currents through a grain boundary. We relate our data to two models, one that assumes acceptor-like traps localized at a grain boundary and another that assumes localized donor-like traps. Using the activation energy as a measure of the potential well (acceptor model) or barrier (donor model) at a grain boundary, we calculate trap densities of the order of 1012cm-2 assuming discrete trap energies of 0.015 and 0.34 eV relative to the valence band in the acceptor and donor models, respectively. We note that the calculated values of the trap density are sensitive to the estimated value of the trap level.