We have investigated the magnetically ordered state of TmSe below 3 K by means of thermal expansion and magnetostriction measurements (down to 1.5 K and up to 15 kOe) as well as by a.c. susceptibility measurements in different magnetic fields. From these data it is possible to construct a diagram of the magnetic phases of low temperatures. The ordering is antiferromagnetic in nature but we find 3 distinctly different phases below the Néel temperature TN. The Néel temperature remains constant in external magnetic fields up to 5 kOe. For higher fields an unambiguous interpretation of the experimental features is difficult.