The (3)Sigma(u)(-)<--X (3)Sigma(g)(-) electronic absorption spectrum of linear C-4 has been detected in the gas phase. The origin and several vibronic transitions have been recorded by means of cavity ring down spectroscopy through a supersonic planar plasma. The origin band is found at 26 384.9(2) cm(-1) (similar to 379 nm). A partly rotationally resolved origin band spectrum yields a value of B-0'=0.1570(5) cm(-1) for the electronically excited (3)Sigma(u)(-) state.