THE transistor, in its various forms, is a three-terminal amplifying electronic device(1). Transistors are usually based on inorganic semiconductors, such as silicon or gallium arsenide(1), but there is increasing interest in the use of organic semiconductors(2-4), motivated by their structural flexibility and tunable electronic properties. The organic transistors fabricated to date have used a conventional ’field-effect’ architecture; unfortunately, such devices involve relatively long conduction pathways which, owing to the low carrier mobilities of the organic materials, render them inherently slow. In an attempt to circumvent this problem, we have developed a different device geometry, more closely related to that of the vacuum-tube triode. The structure consists of a thin film of a semiconducting polymer sandwiched between two electrodes, with the third electrode-a layer of a porous metallic polymer(5)-embedded within the semiconductor. The third electrode plays a role similar to that of the grid in a vacuum tube, controlling the current flow between the two outermost electrodes. This thin-film architecture reduces the length of the conduction pathway, resulting in a relatively fast response time and, in contrast to conventional field-effect transistors, does not require lateral patterning.