In the quest of bright and long persistent far-red/near-infrared phosphors for in vivo optical imaging, the interest in the family of ZnGa 2 O 4 spinel compounds doped with Cr 3+ has been aroused in the most recent years. We show that the dopant concentration plays an important role in the total persistent luminescence output of the material. ZnGa 2 O 4 doped with 0.25%, 0.50% and 0.75% Cr relative to (Ga+Cr) was prepared by solid state synthesis. 0.50% Cr was found optimal to obtain the most intense persistent luminescence after matrix excitation with X-rays or localized excitation in Cr 3+ absorption band with 550nm wavelength. Up to 0.5% Cr content, persistent luminescence increases as a consequence of an increased number of Cr 3+ luminescent centers and associated defects. With 0.75% Cr content, a too large number of defects locally concentrated around Cr 3+ ions are detrimental to the long-term persistent luminescence intensity. We supplement long lasting phosphorescence investigation with laser excited photoluminescence and thermally stimulated luminescence results.