An about 108 ohm-cm AgGaSe2 crystal of 0.5 mm × 4 mm × 4 mm was polished and contacts were made by evaporating 130 μg/cm2 gold of 3 mm diameter on the two faces of the crystal. The detector was tested using 5.5 MeV alpha-particles at room temperature. Noise increased above an applied bias voltage of 80 V. For + 50 V applied on the electrode opposite to the particle incident electrode, i.e., for electron traversal mode, the preamplifier output pulses showed a risetime of 20 μs and an amplitude of about one-tenth of that obtained from a silicon surface-barrier detector whereas the silicon detector showed a risetime of 0.07 μs. For - 50 V applied on the same electrode, i.e., for hole traversal mode, no pulses were observed.