A crystal of BaCaBO3F was grown by the Czochralski method. It shows a phase transition at 242.7 °C upon cooling where the hexagonal high-temperature modification is transformed to a monoclinic low-temperature form (space group C2, a = 17.8779(8) Å, b = 9.0596(4) Å, c = 12.9735(5) Å, β = 118.875(13)°, V = 1840.03(22) Å3, Z = 18). The monoclinic phase has a symmetry lowered by an index of 18 relative to the hexagonal supergroup. This represents a higher symmetry than the rhombohedral model which has a symmetry of index 36 lower than the hexagonal form determined in a previous study (Li and Zeng, J. Cryst. Growth 2013, 382, 47 ). The monoclinic system is confirmed by optical investigations with a polarizing microscope showing biaxial negative character and refractive indices n x = 1.615(1), n y = 1.664(1), and n z = 1.665(1), 2V x = 10.0(2)° as measured on a microrefractometer spindle stage in immersion liquids. Electron microprobe analysis yielded a slight excess of Ba resulting in the composition Ba1.04Ca0.96BO3F corresponding to 7Ba4.7 8Ba6 9Ba4 10­Ba4 7Ca17.3 3B18O54F18 in a crystal-chemical notation for the unit-cell content with the cations in their respective coordination. On the basis of this composition, the mean refractive index can be calculated from individual electronic polarizabilities of the ions resulting in <n calc> = 1.638 which is close to the observed value of <n obs> within an error of 0.5%.