The introduction of deregulated markets requires correct operation strategies for the competitiveness of electricity utilities. Optimum strategies are determined in this paper for different groups of thermal power plants using a dynamic programming technique suggested by the authors in a previous paper. Attention focuses here on the 'Day-Ahead market' session, considering the viewpoint of a company which manages a power plant or a group of power plants trying to maximize profit over the whole session. To cover different situations that could be met in the market, examples are presented both in case of limits imposed on the total production and when all energies produced are supposed to be accepted by the market. (C) 2005 Elsevier Ltd. All rights reserved.