High-pressure resistivity measurements on mixed-valence TmSe indicate a new electronic phase transition near 30 kbar at liquid-helium temperature. In the paramagnetic phase, the ρ-T curve shows unexpectedly small pressure dependence, in contrast with the predictions of a simple valence-change model. The onset of the aniferromagnetic ordering triggers a metal-insulator transition for pressures in the range 0-30 kbar. The new high-pressure phase is characterized by a steep decrease of the resistivity below 3.7 K at 32.5 kbar. © 1980 The American Physical Society.