A model based on the mixing length description is proposed for the prediction of strongly non isothermal subsonic boundary layers. It is validated on a set of experiments involving flows over plates heated up to 1000 K. It is based on the hypothesis that the u＇nu＇ profile is not modified by the heating, at least in the internal part of the flow. Regarding the isothermal case, the calculations lead to an appreciable increase of the skin friction (about 30% at 950 K).