Heat transfer experiments are conducted for fully developed forced and natural flows of water through triangularly arrayed, seven uniformly heated rod bundles with P/D = 1.25, 1.38 and 1.5. For forced circulation experiments. Re ranges from 80 to 50000 and Pr from 3 to 8.5; while in natural circulation. Re varies from 260 to 2000, and Ra, from 8 × 106 to 2.5 × 108. The forced flow data fall into two basic flow regimes : turbulent and laminar flow. At the transition between these regimes, ReT, which varies from 2200 for P/D = 1.25 to 5500 forP/D = 1.5, increases linearly with P/D. The turbulent heat transfer data is in good agreement (±15%) with Weisman's correlation, designed for fully developed turbulent flow in rod bundles at Re > 25 000. However, the laminar flow data shows the dependency of Nu on Re to be weaker than for turbulent flow. Natural circulation data indicates that rod spacing insignificantly affects heat transfer; for P/D = 1.38 and 1.5, Nu is correlated as Nu = 0.272.Ra0.25q.