Fully developed laminar mixed convection flow in a horizontal curved duct of square cross section, with thermal boundary conditions of peripherally uniform wall temperature and axially uniform heat flux is investigated. Numerical solutions are obtained over a range of Dean and Grashoff numbers with mean radius of curvature of 100 and Prandtl number of 0. 73. As one of the driving forces (buoyancy or centrifugal) becomes larger than the other, the friction factor and Nusselt number values exhibit an asymptotic behavior. It is possible to predict the combined effect of these driving forces from the limiting solutions of pure forced convection in curved ducts and mixed convection in straight ducts. At very high flow conditions (Gr greater than 450,000 or Dn greater than 135) a periodic solution evolves.