The birefringence dispersion of a ferroelastic single domain of magnetoelec. BiFeO3 crystal was measured at room temp. (24°) in the rhombohedral phase. A thin (11 μm) (110)cubic cut was used although it was not a principal cut. The optical axis (and spontaneous polarization) formed an angle of 35° with the normal to the cut. Due to anomalous interference colors, the right order was deduced with the help of conoscopy. The birefringence of this cut shows normal dispersion and its value is very large (dn' = 0.13 at λ = 550 nm to 0.08 at λ = 850 nm). The principal birefringence (dn = 0.34 at λ = 550 nm ) was evaluated with the help of the computed (Gladstone-Dale relationship) mean refractive index (̅n =2.62). From dn'(T), (8K < T < 820 K) the Neel temp. was found for the 1st time optically, TN = 653 K (380°), and a crit. exponent was tentatively derived near TN.