Numerical solutions for steady laminar thermally developing flow in a rectangular duct with a sinusoidally corrugated surface are presented. All surfaces are under uniform heat flux conditions (circumferentially and axially). Comparisons are made with previously published results on steady laminar thermally developing flow in a horizontal square duct. In the present problem, a horizontal rectangular duct is obtained by setting the corrugation amplitude aspect ratio and orientation angle to zero. Significant differences in heat transfer rates and temperatures are observed through the ratio of curvature-assisted Nusselt number to flat surface Nusselt number in the developing flow regime.