The first ambipolar light-emitting transistor of an organic molecular semiconductor single crystal, tetracene, is demonstrated. In the device configuration, electrons and holes injected from separate magnesium and gold electrodes recombined radiatively within the channel. By varying the applied voltages, the position of the recombination/emission zone could be moved to any position along the channel. Because of the changes made to the device structure, including, the use of single crystals and polymer dielectric layers and the adoption of an inert-atmosphere fabrication process, the set of materials that can be used for light-emitting transistors has been expanded to include monomeric molecular semiconductors.