A strong anisotropy in the reflectivity measurements of cubic insulators above the absorption edge is observed for what we believe to be the first time. This observed anisotropy in ${\mathrm{Bi}}_{4}$${\mathrm{Ge}}_{3}$${\mathrm{O}}_{12}$ single crystals is ten times stronger than that observed recently in cubic semiconductors. Experimental results are discussed in terms of anisotropy induced by spatial dispersion within the frame of a phenomenological model.