Experimental test results for flow boiling of pure ammonia inside horizontal tubes were obtained for a plain tube and a microfin tube. Tests were run at a nominal saturation temperature of 4°C, mass velocities from 20-140 kg/m2s, vapor qualities from 1-99% and heat fluxes from 7-58 kW/m2. The results showed substantial heat transfer augmentation for the microfin tube at mass velocities below 80 kg/m2s while little or no augmentation was observed at higher mass velocities. Two-phase pressure drops for the microfin tube were typically about 50% higher than for the plain tube at similar test conditions.