The modulational instability development of Langmuir waves is investigated in highly collisional plasmas where the characteristic frequency &OHgr; of the modulated perturbations is much less than &ngr;eff, the effective electron collision frequency. It is demonstrated that the modulational instability for the situation considered is mostly determined by collisional effects (differential Joule heating nonlinearity), in contrast to the well‐known modulational instability in collisionless plasmas (where ponderomotive force nonlinearity dominates). Rates and thresholds of the instability are found in various limits. The modulational instability is most effective when the angles between a pump wave vector and wave vectors of the modulational perturbations are of order unity.