Using hydrodynamic model of a semiconductor-plasma and following the coupled-mode approach, an analytical investigation of excitation of a polaron mode is made in a direct gap 111-V weakly polar, doped semiconductor in presence of an applied magnetostatic field. The effect of magnetostatic fields on the basic operational characteristics of a parametric oscillator, viz., threshold condition and conversion efficiency has been critically analyzed. Numerical estimates are made for an n-InSb crystal at 5K duly irradiated by a pulsed 10.6 tm CO2 laser. The results are found to be in a good qualitative agreement with available experimental observations.