The effect of viscous dissipation on non-Darcy natural convection flow along an isothermal vertical wall embedded in a saturated porous medium is initiated. The flow field is divided into non-Darcy G = 0, intermediate G = 0(1), and limit Darcy regimes G --> infinity using the inertia-buoyancy scales. Thermal dispersion effects are also taken into consideration. It is observed that the viscous dissipation effect reduces the heat transfer rate by about 10% in all three flow regimes. It is also found that the effect of viscous dissipation is greater as the value of the parameter G increases. The effect of viscous dissipation in ail three flow regimes increases with the dispersion parameter.