The higher order zero field steps of long inline and overlap Josephson junctions have been investigated experimentally by means of low temperature scanning electron microscopy. The results indicate that several stable states are possible when more than one fluxon is present in the junction, corresponding to a different spacing among the fluxons during their propagation. These dynamic states should differ by the spectrum of the emitted radiation. Numerical simulations show that the fluxon interaction with plasma oscillations appears to be responsible for the observed behavior. ©1995 American Institute of Physics.