Thermal entrance-length results are presented for the flow of two different fluids, water and oil, in a ¾-in. 4-ft-long tube with a constant heat input per unit length. A hydrodynamic calming section of 97 tube diameters preceded the test section. Pressure-drop measurements in the water tests indicate that the condition of fully established flow at the test-section entrance was closely approximated, if not actually attained. Confidence in the entry-length heat-transfer results is indicated by the agreement of the asymptotic heat-transfer performance of both test fluids with the generally accepted correlation equations. The resulting thermal-entrance lengths were from 10 to 15 tube diameters over the Reynolds modulus range of 104 to 105 and agreed favorably with the analytical results of Deissler and Latzko. When compared at the same Reynolds modulus, the water and oil-entrance lengths were in excellent agreement over the range of Reynolds number common to both fluids (17,000 to 48,000), thereby indicating a negligible influence of the Prandtl modulus on the thermal-entrance length for Prandtl numbers above unity. In the transition region, experimental entry-length values are reported for the flow of oil. As the Reynolds number was increased from 2000 to 10,000, the thermal-entrance length steadily decreased from a large value representative of laminar flow down to a value of approximately 10 diameters.