An intense laser incident on a plasma relativistically modulates the electron mass at the second harmonic. Near the critical layer, the oscillatory mass couples two oppositely propagating Langmuir waves which grow at a rate $\Gamma\simomega_{\rm p} v^2_0/16c^2$, where $omega_{\rm p}$ is the plasma frequency, v0 is the oscillatory electron velocity and c is the velocity of light in vacuum. The decay is accompanied by the excitation of a low-frequency mode which at large wave number $(kc>omega_{\rm p})$ enhances the growth rate of the two-plasmon process.