The electronic properties of the threading dislocations in undoped GaN have been studied using electron holography. The electrostatic potential profiles show that edge, screw, and mixed dislocations are negatively charged. The line charge density at the dislocation core, defined by n electrons per unit cell along the c direction, is determined. The line charge densities for edge, screw, and mixed dislocations are ∼0.3, 1.0, and 0.6 e/c, respectively. The corresponding radii of the charged core are 15, 40 and 20 nm, respectively. A model for the charge distribution around dislocations is discussed.