The FENICE detector, installed at the Frascati e+ e− storage ring ADONE, for measuring the neutron e.m. form factors in the timelike region, is described. FENICE is a nonmagnetic detector and consists of a complex array of scintillators, limited streamer tubes and iron converters for detecting mainly the process e+ e− → nn. Antineutrons are identified by the charged prongs produced in their annihilation and the antineutron velocity is measured by the retrieved annihilation time with respect to the beam crossing time. Neutrons are detected in plastic scintillator layers. To reduce the cosmic rays background a concrete shield, covered by an active veto system, is added.