We present an experimental approach to determine the intrinsic field-effect mobility in an organic single crystal as a function of the in-plane crystal-axis direction. Using a multiterminal geometry the experiment also excludes the effects of the contact resistances on the transport properties. We have applied the method to quaterthiophene thin films. At gate voltages above the threshold voltage, the channel conductance varies linearly with the applied gate voltage. The resulting charge carrier mobility is1.2×10−3&hthinsp;cm2/V&hthinsp;s.This value is constant over a period of weeks of continuous operation and does not vary from sample to sample, in contrast to the typically observed lower mobilities of polycrystalline quaterthiophene thin-film transistor devices. These results emphasize the need for well-characterized model systems to study the intrinsic transport properties of organic materials. ©1998 American Institute of Physics.