A high speed Ga0.51In0.49P/GaAs heterojunction phototransistor grown by metal organic vapour phase epitaxy (MOVPE) is reported. High frequency measurements indicate that the device has a cutoff frequency of 1.7 GHz with a 50 Omega output load. The impulse response of the device is measured to be similar to 60 ps full width at half maximum (FWHM). The phototransistor exhibits an external AC gain of 29 at an incident powter of 500 mu W.