Mixed convection heat transfer from a heated horizontal circular cylinder in Bingham plastic fluids has been studied numerically over wide ranges of the governing parameters as follows: Reynolds number, 0.1 <= Re <= 40; Prandtl number, 1 <= Pr <= 100; Richardson number, 0 <= Ri <= 2 and Bingham number, 0 <= Bn <= 10. Extensive results on the flow and heat transfer characteristics are presented in terms of the streamlines and isotherm contours in the close proximity of the cylinder and the distribution of pressure and the Nusselt number over the surface of the cylinder. The gross behavior is described in terms of the drag coefficient and average Nusselt number as functions of the above-noted influencing parameters. In addition, the morphology of the flow domain in terms of the size and shape of (and their dependence on the governing parameters) the yielded- and unyielded regions separated by the so-called yield surfaces is also analyzed. The momentum and thermal boundary layers progressively thin with the increasing values of each of Re, Pr, Bn and RI. Thus, it stands to reason that the rate of heat transfer should bear a positive dependence on each of these parameters. The results reported herein elucidate this fundamental dependence. Finally, the heat transfer results are consolidated by choosing a slightly modified velocity and viscosity scales thereby enabling a satisfactory correlation between the modified Nusselt number and Reynolds number. (C) 2014 Elsevier B.V. All rights reserved.