Millennium-old natural dye indigo - a "new" ambipolar organic semiconductor. Indigo shows balanced electron and hole mobilities of 1 x 10(-2) cm(2) V(-1) s-1 and good stability against degradation in air. Inverters with gains of 105 in the first and 110 in the third quadrant are demonstrated. Fabricated entirely from natural and biodegradable compounds, these devices show the large potential of such materials for green organic electronics.