The velocity of propagation of the luminous detail in a plasma jet was determined by the time required for transit between photomultiplier stations at different axial positions of an expanded plasma jet. The measurements were synchronized with power supply ripple to eliminate this as a random perturbation.It is found that the luminous fluctuations travel at two velocities. The slower fluctuations move with nearly the calculated speed of the jet and may therefore be ascribed to temperature or possibly composition fluctuations while the (much) faster fluctuations travel with jet speed plus local sonic velocity and are evidently pressure waves.