Hexagonal gallium orthophosphate crystals have been obtained by spontaneous nucleation using the slow cooling method from X2O-3MoO(3) fluxes with X = Li, K. Compared to GaPO4 crystals grown by hydrothermal methods, infrared measurements have revealed flux-grown samples without hydroxyl groups and thermal analyses have pointed out the total reversibility of the phase transition alpha-quartz GaPO4 <-> beta-cristobalite GaPO4. The elastic constants of these millimeter-size flux-grown alpha-GaPO4 piezoelectric crystals were experimentally determined from their Brillouin scattering behaviour at room and high temperatures. The room temperature results were in good agreement with the published ones concerning hydrothermally grown samples and the two longitudinal elastic constants measured versus temperature until 850 degrees C have shown a monotonous evolution. (C) 2007 Elsevier B.V. All rights reserved.