LIF spectra of the A 2 Π i --X 2 Π i transition of the HCCS radical were observed in a supersonic jet generated by using a pulsed-discharge nozzle. Adiabatic cooling yielded definitive rotational assignments for 40 vibronic bands. The fluorescence from single rovibronic levels exhibited a quantum beat in most of the observed bands. The number of beat components increased gradually with internal energy, approaching the limiting case for a large molecule . The observed quantum interference is ascribed, based on the estimated densities of background states, to the internal conversion coupling with highly excited vibrational levels in the X 2 Π i state.