We have recently studied by XRD (at ESRF) and RIXS (at SOLEIL) the pressure and low temperature phase diagram of superconducting (SC) FeSe. The Cmma phase of FeSe transforms at 1GPa in a distorted structure, found antiferromagnetic (AFM) by muons spectroscopy. Above 6.5GPa a second high pressure (HP) phase (Pnma) appears which shows a higher magnetic moment (deduced from the K beta' (Fe) emission line). In this proposal we want to determine the magnetic structure of both HP phases by neutron powder diffraction on D20 instrument using a single phase powder sample obtained from grinding high quality FeSe single crystals and the large volume Paris-Edinburgh cell. We expect:- to resolve for the first time the magnetic structure of the HP orthorhombic (Pnma) phase which appears above 6.5GPa at low T and which is in competition with the SC phase whose Tc decreases for P>6.5GPa- to confirm the distortion of the low pressure phase found in the 1-6.5GPa range at low T and determine its AFM structure.More generally our work will allow to better understand the low T part of the controversial P,T phase diagram of FeSe in particular the interplay between the SC phase and the magnetic ones.