The stability of a finite-amplitude monochromatic Langmuir wave is considered in one dimension. A dispersion relation is obtained which includes the decay, purely growing and modulational instabilities. It is shown that for an infinite-wavelength Langmuir pump wave the modulational and oscillating two-stream instabilities are the same. It is also pointed out that the threshold for the modulational instability is equal to the threshold for the inverse oscillating two-stream instability, in which the Langmuir wave energy is converted into electromagnetic radiation.