For the first time, single crystals of the bismuth(2:1)borate Bi4B2O9 have been grown by the Czocliralski method. The linear optical properties of this monoclinic substance with an exceptionally high density of 8.208 g/cm(3) were investigated. Between crossed polars an extreme dispersion of the extinction angle was detected. By measuring all principal refractive indices the wavelength dependence of the extinction angle could be calculated. (C) 2002 Elsevier Science B.V. All rights reserved.