A thermal instability is studied for the generation of large-scale field-aligned irregularities in the F-region of the ionosphere. These irregularities are then explained to be the cause of the spread-F traces in the ionograms recorded in the recent Arecibo heating experiments. The background plasma is assumed to have an elevated electron temperature and a localized heat source located near the HF reflection height. The elevated electron temperature is maintained by the heat source which is supplied by the HF heater. The dispersion relation of the instability is derived and analyzed for the threshold condition and the growth rate. The calculated growth time of the large-scale irregularities is consistent with the Arecibo observations when measured electron-to-ion temperature ratios in the range between 2 and 3 are used in the analysis. ¿ American Geophysical Union 1988