The authors present a theory for the extremely fast instability of the diamond structure of semiconductors in the presence of a dense electron-hole plasma, which can be generated by an intense laser pulse. They find that the transverse acoustic phonons become unstable if more than 8% of the valence electrons are excited into the conduction band. Furthermore, if 15-20% of the valence electrons are excited, the cubic symmetry of the lattice is destroyed within less than 200 fs after the laser pulse. The crystal itself melts a few ps later. The results are in good agreement with experiments performed on Si and GaAs.