A nonlinear scattering process has been analyzed as a source mechanism producing the frequency up-shifted HF-enhanced plasma lines (HFPLs) observed in the Arecibo ionospheric heating experiments. Parallelly propagating Langmuir waves generated by the parametric decay instability of the HF pump near the reflection height scatter off the background lower hybrid density fluctuations to produce the up-going and down-going Langmuir waves with frequencies greater than the pump frequency by several tens of KHz. The frequency up-shifted nature of excited Langmuir waves stems from the fact that the natural frequencies of the obliquely propagating Langmuir waves are higher than those of the parallelly propagating Langmuir waves at the same altitude. A physical picture is thus offered to explain how Langmuir waves with frequencies greater than the HF heater wave frequency can be produced in the heating experiments and be detected by incoherent radars as frequency up-shifted HFPLs. Since the considered scattering process occurs in a region near the reflection height it explains why the frequency up-shifted HFPLs should originate from the altitude near the reflection height as observed. Moreover, the theory also shows that the amount of frequency up-shift is inversely proportional to the frequency of the HF heater and increases linearly with the electron temperature. The quantitative analysis of the theory shows a good agreement with the experimental results. ¿ American Geophysical Union 1992