Experiments have been performed to study the effect of circumferentially nonuniform heating on the fully developed turbulent heat transfer characteristics for water flow in a horizontal circular tube. The use of a specially fabricated tube enabled heat to be supplied to the fluid over half its circumference, while the other half was unheated. Separate sets of experiments were conducted with the heated portion at the top and at the bottom. By varying the temperature level, the Prandtl number was varied from 3.5 to 11.5. The Reynolds number ranged from 3,000 to 70,000. The measurements enabled circumferential distributions of the Nusselt number, wall temperature, and heat flux to be determined, and circumferential average Nusselt numbers were also evaluated. Both the circumferential average and circumferential local results demonstrate that significant buoyancy effects are present for bottom heating at low Reynolds numbers and high Rayleigh numbers, and a criterion is deduced for the onset of these effects. The top heating experiments were not affected by buoyancy. The buoyancy-unaffefted circumferential average Nusselt numbers increase smoothly over the entire range of Reynolds numbers, and the Prandtl number dependence is correlated as Pr0.47. These Nusselt numbers are within about ten percent of literature correlations (applicable for Re ≥ 10,000) for circumferentially uniform thermal conditions. The circumferential distributions of the Nusselt number and temperature on the heated wall tend toward uniformity when the turbulence is well developed (i.e., higher Reynolds numbers) or when buoyancy is present. The temperatures on the unheated wall are generally lower than the bulk temperature in the absence of buoyancy, but, when buoyancy is active, they are above the bulk temperature.