Numerical simulations of thermal convection in a wide (8¿8¿1) Cartesian box heated from below with temperature-dependent viscosity contrasts of 1000, and Rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern. With rigid upper and lower boundaries, spoke-pattern flow with small (diameter ~1.5) cells is obtained, consistent with laboratory experiments and previous numerical results. However, with the arguably more realistic stress-free boundaries, the flow chooses the largest possible wavelength, forming a single square cell of aspect ratio 8, with one huge cylindrical downwelling surrounded by upwelling sheets. The addition of stress-dependence to the rheology weakens the stiff upper boundary layer, resulting in smaller cells, though still with upwelling sheets and downwelling plumes. ¿ American Geophysical Union 1993