The impurity effect on the charge carrier transport in a smectic liquid crystal was investigated by time-of-flight experiments with a model system of a 2-phenylnaphthalene derivative of 2-(4'-octylpheny)-6-dodecyloxynaphthalene (8-PNP-O12) and a neutral impurity of dihexylterthiophene that can be a chemical impurity for hole and electron. In contrast to ionic conduction in the isotropic phase of doped 8-PNP-O12, two-dimensional trap-controlled electronic conduction took place in the SmB phase, and the electronic and ionic conductions coexisted in the SmA phase and the electronic conduction vanished when the impurity concentration was increased over 50 ppm. (C) 2004 Elsevier B.V. All rights reserved.