Organic field-effect transistors (OFETs, see Figure), based on the solution-processible methanofullerene [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM), have been fabricated in a bottom-contact device configuration using gold electrodes. The OFET functions either as a p-or n-channel device, depending upon the bias conditions. This is the first example of ambipolar charge transport in FETs based on pristine PCBM.