[en] Specific heat and neutron diffraction experiments on polycrystalline samples as well as magnetization measurements on a single crystal of the hexagonal HoAlGa compound are presented. Below T_N=31 K, two magnetic phases are present with the Ho moments along c. The low temperature equal moment antiferromagnetic structure characterized by the propagation vectors (1/3,1/3,1/2) and (0,0,1/2) transforms, above T_t=18.5 K, toward a sine wave modulated structure with an incommensurate propagation vector (1/3,1/3,0.481). Surprisingly at T_t no anomaly can be detected by specific heat and by susceptibility. Along c, multistep metamagnetic behaviours (three transitions at 1.8 K) are observed from which the H-T phase diagram has been determined. (orig.)