In this study, two-dimensional steady-state simulations of laminar natural convection in square enclosures with differentially heated sidewalls have been carried out where the enclosures are considered to be completely filled with a yield stress fluid obeying the Bingham model. Yield stress effects on heat and momentum transport are investigated for nominal values of Rayleigh number (Ra) in the range 10(3)-10(6) and a Prandtl number (Pr) range of 0.1-100. It is found that the mean Nusselt number (Nu) over bar increases with increasing values of Rayleigh number for both Newtonian and Bingham fluids. However, (Nu) over bar values obtained for Bingham fluids are smaller than that obtained in the case of Newtonian fluids with the same nominal value of Rayleigh number Ra due to weakening of convective transport. The mean Nusselt number (Nu) over bar in the case of Bingham fluids is found to decrease with increasing Bingham number, and, for large values of Bingham number Bit, the value settles to unity ((Nu) over bar = 1.0) as heat transfer takes place principally clue to thermal conduction. The effects of Prandtl number have also been investigated in detail and physical explanations are provided for the observed behaviour. New correlations are proposed for the mean Nusselt number (Nu) over bar for both Newtonian and Bingham fluids which are shown to satisfactorily capture the correct qualitative and quantitative behaviour of (Nu) over bar in response to changes in Ra, Pr and On. (C) 2010 Elsevier B.V. All rights reserved.