Al0.5Ga0.5As/GaAs heterojunction phototransistors have been fabricated from structures grown by the MO-CVD process. The relatively high optical gains (∼100) and short response times (≲2 nsec) obtained with these devices indicate that an optimized AlxGa 1-xAs/GaAs heterostructure phototransistor could be a suitable detector for optical-fiber-communication systems using GaAs DH lasers.