A numerical study of the laminar mixed convection of micropolar fluids in a square cavity with localized heat source is presented. The flow in the cavity includes the externally induced :stream at ambient temperature and the free flow induced by heat source. The transport equations of vorticity, angular momentum, and energy are solved with the aid of the cubic spline collocation method. The computation is carried out for wide ranges of Reynolds number, Grashof number, and various material parameters associated with the micropolar fluids. Emphasis is aimed at the significant features displayed by the micropolar fluids. The results indicate that both the flow and the thermal fields depend on the vortex viscosity and spin gradient viscosity to a large extent. The heat transfer coefficient of micropolar fluids is found to be smaller than that of Newtonian fluids. (C) 2000 Elsevier Science Ltd. All rights reserved.