We report the first neutron diffraction study of the magnetic ordering in stoichiometric TmSe under pressures up to 20 kbar. The most striking result is the persistence of the type-I antiferromagnetic structure. We have observed the pressure dependence of the magnetic moment and of the Néel temperature; the temperature dependence of the sublattice magnetization (in reduced units) is independent of pressure. Comparisons are made with abnormal cerium compounds. © 1980.