The effect of viscous dissipation oil parallel Darcy flow in a horizontal porous layer with an adiabatic lower boundary and an isothermal upper boundary is discussed. The presence of viscous dissipation serves to cause a nonlinear temperature profile within the layer. The linear stability of this nonisothermal base flow is then investigated with respect to the onset of convective rolls. The solution of the linear equations for the perturbation waves is determined analytically by a power series method, and the results are confirmed using a direct numerical approach Using a fourth order Runge Kutta method. The neutral stability curve and the critical value of the governing parameter R = GePe(2) are obtained, where Ge is the Gebhart number and Pe is the Peclet number. The effect of an imperfect isothermal boundary condition at the upper boundary is investigated by considering finite values of the Biot number. (c) 2008 Elsevier Ltd. All rights reserved.