Persistent post-photocurrents (PPC) have been studied in organic field-effect transistors (FET) made of thin films of regioregular poly(3-octylthiophene). The phenomenon occurs after illumination of FET with UV light. The drain-source current persisting after switching off the illumination is ca. three orders of magnitude higher than that in a virgin sample, and lasts for a long time. It may be erased by applying a negative potential to the gate electrode or by reversing the drain-source voltage. The magnitude and kinetic behaviour of the current depend on ambient atmosphere, particularly on the presence of O2 molecules. The effect has been explained by formation of mobile anions during irradiation. These slowly recombining anions are responsible for the occurrence of PPC. The effect may be used in opto-electronic switches.