Measurements of indeces of refraction,birefringence,rotation of optical ellipsoid and optical activity have been made near incommensurate phase transition temperature and near two- and three- dimensional magnetic ordering tertpera- tures in BaMnF. The results are compatible with a single first order structural phase transition at T = 251 K. The point group symmetry below T is shown to be 2[C} or lower. Magnetic contribution to the refractive index at low m - 3temperatures is obtained, the highest value beingoHI 10.