The multiplicity of convection patterns and the effect of parameters such as wavenumber, aspect ratio, Pr and non-Boussinesq conditions have generally complicated the study of heat transfer in Rayleigh-Benard convection. In this paper numerical heat transfer results are described for problems in a cavity with horizontal aspect ratios of 4.5×4.5 having symmetry boundary conditions imposed on the lateral walls. Boussinesq fluids and variable viscosity fluids with exponential, linear and inverse viscosity functional relations are considered. Solutions have been obtained for viscosity ratios as high as 50. The results for the variable viscosity cases are presented in terms of a Rayleigh number that is based on the thermally averaged viscosity, Raavt, viscosity ratio, r, and deviation ratio, rd.