Ultraviolet Al0.17Ga0.83N∕GaN-based heterojunction phototransistors (HPTs) grown by metal-organic vapor-phase epitaxy were demonstrated. The HPTs showed a bandpass spectral responsivity ranging from 280to390nm. With a bias voltage of 6V, the responsivity at an incident of 340nm was as high as 1500A∕W, corresponding to a quantum gain of 5.47×103. In contrast to GaN-based photoconductors, the HPTs also featured high contrast in spectral response. With a bias voltage of 3V, the spectral response showed high rejection ratios of approximately 4×105 and 1×104 for the long-wavelength side (340∕400nm) and the short-wavelength side, respectively. The high contrast of spectral response for the long-wavelength side could be due to the long trapping time of holes blocked by the base-emitter heterojunction and the low defect-to-band response that is caused by defect-related gap states.