A two-dimensional numerical simulation is conducted to analyze the steady laminar natural convective flow and heat transfer of micropolar fluids in a square enclosure. The vertical walls are kept at isothermal conditions, while horizontal walls are assumed to be insulated. Employing the finite difference method, computations are carried out to investigate the material parameter of the micropolar fluid, the Rayleigh and Prandtl numbers, both for weak and strong concentration cases. It was shown that micropolar fluids give lower heat transfer values than those of the Newtonian fluids. An increase of the material parameter, K is shown to decrease the heat transfer. The results for K = 0, which corresponds to the Newtonian fluid case are compared with those available in the existing literature, and an excellent agreement was obtained. (c) 2006 Elsevier Masson SAS. All rights reserved.