Magnetic measurements on nearly stoichiometric TbN single crystals reveal metamagnetic behavior with a TN of 31 K. Excess carriers due to nonstoichiometry make the crystals ferromagnetic with TC = 48 K, a fact which has been revealed by elastic neutron scattering measurements in zero applied field. The compound is a self-compensated semimetal, in which the valence p band of nitrogen has a maximum at the Γ point of the Brillouin zone and the conduction d band has a minimum at the X point. A slight overlap of both bands yields about 1021 carriers cm−3. The optical reflectivity and the thermal expansion coefficient have been measured on the ferromagnetic compositions. The plasma edge of the free carriers exhibits a blue shift upon cooling below TC, following a spin correlation function. This is mainly due to an exchange splitting of the conduction d band.