Natural convection in a horizontal annulus with a constant heat flux wall is investigated for the fluids of 0.2 less than or equal to Pr less than or equal to 1. The outer cylinder is kept at a constant temperature, and the inner cylinder is heated with a constant heat flux. By using a numerical approach in solving the unsteady governing equations of flow and temperature fields, it is shown that dual steady solutions exist above a critical Rayleigh number. (C) 2003 Elsevier Science Ltd. All rights reserved.