An experimental study of plasma bi-refringence (Cotton-Mouton effect) in the FT plasma carried out by using an HCN laser (337 μm) is reported. The experiment is intended to test the feasibility of polarimetric plasma density measurements in high-toroidal-field tokamaks. The results show that for a plasma density of ne = 1014 cm3 and a toroidal field of BT = 8 T, the effect of plasma bi-refringence is very strong and corresponds to the theoretical expectation. The discrepancy between experimental and computed data is mainly due to refractive power losses of the HCN beam.