Numerical solutions in boundary-fitted coordinates are presented for the problem of thrcedimensional natural convection in an enclosed vertical rod bundle. The rod bundle is composed of seven hot rods placed in an isothermal cold hexagonal enclosure with the rod surfaces heated under uniform heat flux conditions. Flow fields and temperature distributions are obtained for Rayleigh numbers up to 10" and a Prandtl number of 0.7; the aspect ratio of vertical to horizontal dimensions is set at the value of unity. The distributions of velocities and isotherms are presented. The effect of natural convection on local and average boundary temperatures as well as on heat transfer coefficients is examined. With an increase in the Rayleigh number, the rods tend to achieve nearly the same average boundary temperature, or consequently the same overall heat transfer coefficient. It is demonstrated that in the boundary-layer regime, the Nusselt and Rayleigh numbers can well be correlated with Nu = cRu"~, which coincides with that for a vertical rectangular channel. The results are compared with those for an isothermal rod bundle to examine the effect of boundary conditions on the flow distribution and heat transfer.