  The PADME experiment at the DA$\Phi$NE Beam-Test Facility (BTF) aims at
searching for invisible decays of the dark photon by measuring the final state
missing mass in the process $e^+e^- \to \gamma+ A'$, with $A'$ undetected. The
measurement requires the determination of the 4-momentum of the recoil photon,
performed using a homogeneous, highly segmented BGO crystals calorimeter. We
report the results of the test of a 5$\times$5 crystals prototype performed
with an electron beam at the BTF in July 2016.
