A mechanism for generating the small-scale field-aligned irregularities observed in ionospheric heating experiments is proposed. The Langmuir waves that result from the scattering of the electromagnetic pump wave of ordinary polarization by the irregularities, interact with the irregularities by secondary scattering processes. The differential dissipation of the resulting Langmuir waves causes the growth of the irregularities and of the Langmuir waves resulting from the secondary scattering. The instability is explosive in nature and initial irregularities whose fractional density perturbations exceed about 10-4, are needed for its occurrence. It is shown for a periodically varying field-aligned density perturbation that although successive scattering of the Langmuir waves eventually becomes very intense and approaches an equipartition of the energy among the waves, it doen not saturate the instability. It seems that the proposed mechanism is able to generate small-scale field-aligned irregularities that can reach the amplitudes observed in ionospheric heating experiments.