To investigate the growth of instabilities in an underdense plasma, a number of simulations was carried out using the one‐dimensional electromagnetic (EM) relativistic particle‐in‐cell code. A new type of Raman‐like scattering was identified in a subcritical regime, which is overdense for standard SRS. This novel instability is a parametric decay of the relativistic EM wave into a scattered light and an electron‐acoustic (&ohgr; < &ohgr;p) electrostatic wave. In the linear stage, resonant matchings are well satisfied, while the scattered Stokes wave is always driven near critical. During nonlinear saturation, due to rapid growth and strong localization of the Stokes wave, narrow intense EM soliton‐like structures with down‐shifted laser light trapped inside are formed; eventually, to be irradiated through the plasma‐vacuum interface in the form of intense low‐frequency EM bursts. This behavior alters the distribution of laser energy between transmission, scattering losses and generation of energetic electrons. © 2004 American Institute of Physics